Move files for humans to non-hidden docs/ dir

.github should be used for files targeted at machines and automation.
the CONTRIBUTING file is special and needs to be in a location for
github to find it (as there are various UI locations that file is linked
and referenced). But the _primary_ consumer of that file should be
humans, so let's surface it in a non-hidden directory.

https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

Author: jasonkarns