Move examples out of crates/matrix-sdk

[jplatte]

Is your feature request related to a problem? Please describe.

It's somewhat annoying to have to navigate to crates/matrix-sdk/examples instead of just examples, plus this nesting makes the examples much harder to discover. Also, they don't have individual lists of dependencies, making it harder to use them as a starting point for new projects.

Describe the solution you'd like

Let's make the examples their own crates, same as in Ruma, axum and others.

We should think about whether it makes sense to have them be part of the regular Cargo workspace (like in Ruma) or as a separate Cargo workspace (like in axum). Context for why this is separated in axum: tokio-rs/axum#978