Document RapierContext resource for Bevy plugin

[Jondolf]

What problem does this solve or what need does it fill?

The RapierContext resource is quite a critical part of bevy_rapier. It contains all of the simulation structures, rigid body and collider sets, and IntegrationParameters (also related: #91). It is also used for scene queries and for getting collider parents and contact pairs, and I believe it can even be used to manually step the simulation. Despite all this, it is not mentioned anywhere in the bevy_rapier usage docs except that it is used for scene queries.

It should really be documented to at least mention the things that it contains, and all of the core tasks that it can be used for, along with code examples.

What solution would you like?

Document RapierContext on the website. This could be a part of a "Simulation structures" section similar to the core Rapier, but with more code examples and concrete usage docs, or it could have its completely own section.

[Vrixyz]

Definitely needed, but we might want to wait for dimforge/bevy_rapier#502 to be adressed