docs: move markdown docs from .github/ to docs/

docs: move markdown docs from .github/ to docs/

The GitHub guide on contributing file says: "Decide whether to store your
contributing guidelines in your repository's root, docs, or .github directory."

https://help.github.com/articles/setting-guidelines-for-repository-contributors/#adding-a-contributing-file

But there's really no advantage to keeping it in the hidden .github/, since
these are public and really belong together with the other documentation.

We can still keep the issue templates under .github/, since they are not really
documentation on their own.

Updated the links pointing to CONTRIBUTING.md to refer to the one in docs/.