Update Contributing guide generally and for all org repos

[TimMonko]

📚 New content request

napari/napari-metadata#82 (comment)

The contributing guide is slightly slimmed down compared to napari-plugin-manager, but follow-ups should add an UPDATED contributing guide to all our repos as a maintenance thing, with preference just to link to an UPDATED napari.org guide :)

Outline

1. Have a "beginner's" contributing guide in docs with link to external resources for things ilke getting started with git/github, pyopensci, etc
2. Have the "typical" contributing.md style guide as we do now (but spruced / cleaned up)
3. Link both the guides in a Contributing.md in each repo (so we don't have to do per repo maintenance as we do now)

p.s. there isn't even a contributing.md in napari/napari so right now it harvests the .github/contributing.md 😬