Add docs.rs documentation

[mbrea-c]

Currently the docs.rs page is pretty barren. before leaving alpha this needs to be populated with enough information for users to get started.

TODO:

• Minimal examples
  • How to use graph animation player, and how to replace bevy's own animation players is specific entities.
  • Graphs as assets
  • Programmatically creating graphs with API (Won't do in this PR)
  • Graph with custom node (Won't do in this PR)
• Explanation of how the animation graph works.
• Documentation of available nodes with inputs and outputs specified. This should be linked from front page. (Inputs and outputs will be documented in future patch).