Add a bevy dependency graph to the contributor book (that stays up to date)

Using https://github.com/jplatte/cargo-depgraph, we can have the main bevy repo run a CI task on `main` whenever a new PR is merged that updates the dependency graph and stores it somewhere, then we simply link this standard location as an image in the contributor book.

This would help give a visual of the higher to lower level APIs and crates that bevy provides.

Start by reading the contributor book structure and the existing CI workflows, then inspect how the main Bevy repository could run cargo-depgraph after merges. Done means the dependency graph is generated to a stable location and linked as an image in the contributor book, with later merged changes updating it automatically.

Written by the indexing model from the issue text.