Fix up list formatting and expand adoption section in contributing guide

[rparrett]

There's some pretty funky stuff going on in this document with lists. The "Contributing Code" section in particular is written as a nested numbered list, but the second layer of nesting is not really a series of numbered steps. One of them only has one "sub step."

This PR:

• Flattens some numbered sub-lists into sub-paragraphs.
• Marks the fast-compiles steps as optional (I wouldn't mind deleting this step though)
• Does some other misc tidying in the Contributing Code section
• Expands the content in the adoption section, hopefully hitting the main points of Provide more guidance for PR-adopters in CONTRIBUTING bevy#12160 without going too crazy.

Here's a before / after for the number list formatting.