Add doc-writing advice to Contributing guide

Initial notes from Discord:

1. The first line of doc comments should be a minimal description.
2. Details of what the documented function / type do belong in their own paragraphs, separated by a line break.
3. If the usage is not immediately clear, or if there are subtle gotchas, add doc tests
4. If a method panics, mention that under a Warning header.
5. If a method or struct is unsafe, you must include a Safety header
6. Link to related types that are commonly used with or are alternatives to the documented type using [`DocLinks`].

These should be stored in a sibling to https://github.com/bevyengine/bevy/blob/main/.github/contributing/engine_style_guide.md, which should be linked in https://github.com/bevyengine/bevy/blob/main/CONTRIBUTING.md