Add contributor guide to GitHub's interface

[wwarriner]

Let's make our contributor guide visible to GitHub's interface by following these instructions: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors.

We have a docs directory due to the nature of our repository. We can put the contributor guide at docs/CONTRIBUTING. This may require dropping the .md suffix. If so, we'll want to keep the content in its original location and provide links to relevant sections using an FAQ style.