Improve entity and component API docs (bevyengine#4767)

# Objective

The descriptions included in the API docs of `entity` module, `Entity` struct, and `Component` trait have some issues:
1. the concept of entity is not clearly defined,
2. descriptions are a little bit out of place,
3. in a case the description leak too many details about the implementation,
4. some descriptions are not exhaustive,
5. there are not enough examples,
6. the content can be formatted in a much better way.

## Solution

1. ~~Stress the fact that entity is an abstract and elementary concept. Abstract because the concept of entity is not hardcoded into the library but emerges from the interaction of `Entity` with every other part of `bevy_ecs`, like components and world methods. Elementary because it is a fundamental concept that cannot be defined with other terms (like point in euclidean geometry, or time in classical physics).~~ We decided to omit the definition of entity in the API docs ([see why]). It is only described in its relationship with components.
2. Information has been moved to relevant places and links are used instead in the other places.
3. Implementation details about `Entity` have been reduced.
4. Descriptions have been made more exhaustive by stating how to obtain and use items. Entity operations are enriched with `World` methods.
5. Examples have been added or enriched.
6. Sections have been added to organize content. Entity operations are now laid out in a table.

### Todo list

- [x] Break lines at sentence-level.

## For reviewers

- ~~I added a TODO over `Component` docs, make sure to check it out and discuss it if necessary.~~ ([Resolved])
- You can easily check the rendered documentation by doing `cargo doc -p bevy_ecs --no-deps --open`.

[see why]: bevyengine#4767 (comment)
[Resolved]: bevyengine#4767 (comment)

crates/bevy_ecs/src/component.rs

@@ -14,26 +14,102 @@ use std::{
     mem::needs_drop,
 };
 
-/// A component is data associated with an [`Entity`](crate::entity::Entity). Each entity can have
-/// multiple different types of components, but only one of them per type.
+/// A data type that can be used to store data for an [entity].
 ///
-/// Any type that is `Send + Sync + 'static` can implement `Component` using `#[derive(Component)]`.
 ///
-/// In order to use foreign types as components, wrap them using a newtype pattern.
+/// `Component` is a [derivable trait]: this means that a data type can implement it by applying a `#[derive(Component)]` attribute to it.
+/// However, components must always satisfy the `Send + Sync + 'static` trait bounds.
+///
+/// [entity]: crate::entity
+/// [derivable trait]: https://doc.rust-lang.org/book/appendix-03-derivable-traits.html
+///
+/// # Examples
+///
+/// Components can take many forms: they are usually structs, but can also be of every other kind of data type, like enums or zero sized types.
+/// The following examples show how components are laid out in code.
+///
+/// ```
+/// # use bevy_ecs::component::Component;
+/// # struct Color;
+/// #
+/// // A component can contain data...
+/// #[derive(Component)]
+/// struct LicensePlate(String);
+///
+/// // ... but it can also be a zero-sized marker.
+/// #[derive(Component)]
+/// struct Car;
+///
+/// // Components can also be structs with named fields...
+/// #[derive(Component)]
+/// struct VehiclePerformance {
+///     acceleration: f32,
+///     top_speed: f32,
+///     handling: f32,
+/// }
+///
+/// // ... or enums.
+/// #[derive(Component)]
+/// enum WheelCount {
+///     Two,
+///     Three,
+///     Four,
+/// }
+/// ```
+///
+/// # Component and data access
+///
+/// See the [`entity`] module level documentation to learn how to add or remove components from an entity.
+///
+/// See the documentation for [`Query`] to learn how to access component data from a system.
+///
+/// [`entity`]: crate::entity#usage
+/// [`Query`]: crate::system::Query
+///
+/// # Choosing a storage type
+///
+/// Components can be stored in the world using different strategies with their own performance implications.
+/// By default, components are added to the [`Table`] storage, which is optimized for query iteration.
+///
+/// Alternatively, components can be added to the [`SparseSet`] storage, which is optimized for component insertion and removal.
+/// This is achieved by adding an additional `#[component(storage = "SparseSet")]` attribute to the derive one:
+///
 /// ```
 /// # use bevy_ecs::component::Component;
+/// #
+/// #[derive(Component)]
+/// #[component(storage = "SparseSet")]
+/// struct ComponentA;
+/// ```
+///
+/// [`Table`]: crate::storage::Table
+/// [`SparseSet`]: crate::storage::SparseSet
+///
+/// # Implementing the trait for foreign types
+///
+/// As a consequence of the [orphan rule], it is not possible to separate into two different crates the implementation of `Component` from the definition of a type.
+/// This means that it is not possible to directly have a type defined in a third party library as a component.
+/// This important limitation can be easily worked around using the [newtype pattern]:
+/// this makes it possible to locally define and implement `Component` for a tuple struct that wraps the foreign type.
+/// The following example gives a demonstration of this pattern.
+///
+/// ```
+/// // `Component` is defined in the `bevy_ecs` crate.
+/// use bevy_ecs::component::Component;
+///
+/// // `Duration` is defined in the `std` crate.
 /// use std::time::Duration;
+///
+/// // It is not possible to implement `Component` for `Duration` from this position, as they are
+/// // both foreign items, defined in an external crate. However, nothing prevents to define a new
+/// // `Cooldown` type that wraps `Duration`. As `Cooldown` is defined in a local crate, it is
+/// // possible to implement `Component` for it.
 /// #[derive(Component)]
 /// struct Cooldown(Duration);
 /// ```
-/// Components are added with new entities using [`Commands::spawn`](crate::system::Commands::spawn),
-/// or to existing entities with [`EntityCommands::insert`](crate::system::EntityCommands::insert),
-/// or their [`World`](crate::world::World) equivalents.
-///
-/// Components can be accessed in systems by using a [`Query`](crate::system::Query)
-/// as one of the arguments.
 ///
-/// Components can be grouped together into a [`Bundle`](crate::bundle::Bundle).
+/// [orphan rule]: https://doc.rust-lang.org/book/ch10-02-traits.html#implementing-a-trait-on-a-type
+/// [newtype pattern]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-the-newtype-pattern-to-implement-external-traits-on-external-types
 pub trait Component: Send + Sync + 'static {
     type Storage: ComponentStorage;
 }

crates/bevy_ecs/src/entity/mod.rs

@@ -1,27 +1,38 @@
 //! Entity handling types.
 //!
-//! In Bevy ECS, there is no monolithic data structure for an entity. Instead, the [`Entity`]
-//! `struct` is just a *generational index* (a combination of an ID and a generation). Then,
-//! the `Entity` maps to the specific [`Component`s](crate::component::Component). This way,
-//! entities can have meaningful data attached to it. This is a fundamental design choice
-//! that has been taken to enhance performance and usability.
+//! An **entity** exclusively owns zero or more [component] instances, all of different types, and can dynamically acquire or lose them over its lifetime.
+//!
+//! See [`Entity`] to learn more.
+//!
+//! [component]: crate::component::Component
 //!
 //! # Usage
 //!
-//! Here are links to the methods used to perform common operations
-//! involving entities:
+//! Operations involving entities and their components are performed either from a system by submitting commands,
+//! or from the outside (or from an exclusive system) by directly using [`World`] methods:
+//!
+//! |Operation|Command|Method|
+//! |:---:|:---:|:---:|
+//! |Spawn a new entity|[`Commands::spawn`]|[`World::spawn`]|
+//! |Spawn an entity with components|[`Commands::spawn_bundle`]|---|
+//! |Despawn an entity|[`EntityCommands::despawn`]|[`World::despawn`]|
+//! |Insert a component to an entity|[`EntityCommands::insert`]|[`EntityMut::insert`]|
+//! |Insert multiple components to an entity|[`EntityCommands::insert_bundle`]|[`EntityMut::insert_bundle`]|
+//! |Remove a component from an entity|[`EntityCommands::remove`]|[`EntityMut::remove`]|
 //!
-//! - **Spawning an empty entity:** use [`Commands::spawn`](crate::system::Commands::spawn).
-//! - **Spawning an entity with components:** use
-//!   [`Commands::spawn_bundle`](crate::system::Commands::spawn_bundle).
-//! - **Despawning an entity:** use
-//!   [`EntityCommands::despawn`](crate::system::EntityCommands::despawn).
-//! - **Inserting a component to an entity:** use
-//!   [`EntityCommands::insert`](crate::system::EntityCommands::insert).
-//! - **Adding multiple components to an entity:** use
-//!   [`EntityCommands::insert_bundle`](crate::system::EntityCommands::insert_bundle).
-//! - **Removing a component to an entity:** use
-//!   [`EntityCommands::remove`](crate::system::EntityCommands::remove).
+//! [`World`]: crate::world::World
+//! [`Commands::spawn`]: crate::system::Commands::spawn
+//! [`Commands::spawn_bundle`]: crate::system::Commands::spawn_bundle
+//! [`EntityCommands::despawn`]: crate::system::EntityCommands::despawn
+//! [`EntityCommands::insert`]: crate::system::EntityCommands::insert
+//! [`EntityCommands::insert_bundle`]: crate::system::EntityCommands::insert_bundle
+//! [`EntityCommands::remove`]: crate::system::EntityCommands::remove
+//! [`World::spawn`]: crate::world::World::spawn
+//! [`World::spawn_bundle`]: crate::world::World::spawn_bundle
+//! [`World::despawn`]: crate::world::World::despawn
+//! [`EntityMut::insert`]: crate::world::EntityMut::insert
+//! [`EntityMut::insert_bundle`]: crate::world::EntityMut::insert_bundle
+//! [`EntityMut::remove`]: crate::world::EntityMut::remove
 mod map_entities;
 mod serde;
 
@@ -35,15 +46,57 @@ use std::{
     sync::atomic::{AtomicI64, Ordering},
 };
 
-/// Lightweight unique ID of an entity.
+/// Lightweight identifier of an [entity](crate::entity).
+///
+/// The identifier is implemented using a [generational index]: a combination of an ID and a generation.
+/// This allows fast insertion after data removal in an array while minimizing loss of spatial locality.
+///
+/// [generational index]: https://lucassardois.medium.com/generational-indices-guide-8e3c5f7fd594
+///
+/// # Usage
+///
+/// This data type is returned by iterating a `Query` that has `Entity` as part of its query fetch type parameter ([learn more]).
+/// It can also be obtained by calling [`EntityCommands::id`] or [`EntityMut::id`].
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// #
+/// fn setup(mut commands: Commands) {
+///     // Calling `spawn` returns `EntityCommands`.
+///     let entity = commands.spawn().id();
+/// }
+///
+/// fn exclusive_system(world: &mut World) {
+///     // Calling `spawn` returns `EntityMut`.
+///     let entity = world.spawn().id();
+/// }
+/// #
+/// # bevy_ecs::system::assert_is_system(setup);
+/// # bevy_ecs::system::IntoExclusiveSystem::exclusive_system(exclusive_system);
+/// ```
+///
+/// It can be used to refer to a specific entity to apply [`EntityCommands`], or to call [`Query::get`] (or similar methods) to access its components.
 ///
-/// Obtained from [`World::spawn`](crate::world::World::spawn), typically via
-/// [`Commands::spawn`](crate::system::Commands::spawn). Can be stored to refer to an entity in the
-/// future.
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// #
+/// # #[derive(Component)]
+/// # struct Expired;
+/// #
+/// fn dispose_expired_food(mut commands: Commands, query: Query<Entity, With<Expired>>) {
+///     for food_entity in query.iter() {
+///         commands.entity(food_entity).despawn();
+///     }
+/// }
+/// #
+/// # bevy_ecs::system::assert_is_system(dispose_expired_food);
+/// ```
 ///
-/// `Entity` can be a part of a query, e.g. `Query<(Entity, &MyComponent)>`.
-/// Components of a specific entity can be accessed using
-/// [`Query::get`](crate::system::Query::get) and related methods.
+/// [learn more]: crate::system::Query#entity-id-access
+/// [`EntityCommands::id`]: crate::system::EntityCommands::id
+/// [`EntityMut::id`]: crate::world::EntityMut::id
+/// [`EntityCommands`]: crate::system::EntityCommands
+/// [`Query::get`]: crate::system::Query::get
 #[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
 pub struct Entity {
     pub(crate) generation: u32,