[FEAT] TurboModules guide (#3168)

* [Feat] Add intro for Fabric Components

* feat: add guide to create a Fabric Component

* Beginning of guide/folder structure

* WIP JS Spec

* specification section

* Configuration

* native code intro

* Must be named Spec

* Best stab at iOS native code, but I don't know how to describe what's going on in the code very well. Extrapolated what I could.

* Android instructions

iOS isn't working for me. Builds, but can't load module.

Writing up Android auto-linking next because the steps I tested did
work.

* Include linking instructions from RNNArch repo

* Add example JavaScript

* native modules link

* Address quick feedback items

* Remove, fix for rebased branch

* fix TM parameter on Android

* Revert to 'Codegen' casing

* Revert folly version change

2021.07.22 is for current version on main

* fix typo

* getTurboModule explainer

* Sentence edits

- Fix acronym bolding
- Change wording to "recommended" because "standard" has other
  connotations of possibly being required
- Parentheses unnecessary, distracting

* Remove TODO for now

Getting inconsistent results here, not sure if this is wrong or not;
removing TODO for now so it doesn't block anything

* ABI rephrase, more in line with new Fabric guide wording

* Explain shared C++ code more

* feat: add guide to create a Fabric Component

* feat: add guide to create a Fabric Component

* package.json description

* Lint fixes

* fix: Move JS constants to reduce changes

* fix: Remove newline

* feat: add required step for Android Codegen

* fix: use the proper links

Co-authored-by: Riccardo Cipolleschi <cipolleschi@fb.com>