Replace Contributing in docs w/include to file in repo root.

[slaarti]

Description

I noticed that docs/community/CONTRIBUTING.md not only didn't match the CONTRIBUTING.md that's in the repo root, it's actually just the original "FastAPI-boilerplate" file.

Changes

Replace the contents of docs/community/CONTRIBUTING.md with a snippet include, so that there's only one canonical source which is the one in the repo root.

Checklist

• [X] I have read the CONTRIBUTING document.
• [X] My code follows the code style of this project.
• [X] I have added necessary documentation (if appropriate).