Move the `docs/contributing.rst` contents to `CONTRIBUTING.rst`

[kurtmckee]

This change makes it easier for people to contribute to the project when opening a PR.

Currently, GitHub shows links to CONTRIBUTING.rst when opening a PR, but when people click on those links they are simply told "Go to a third-party website to view the contributing guidelines":

This change replaces the manual redirect with actual content so that no third-party website (in this case, Read the Docs) is necessary to understand the contributing guidelines.

In addition, docs/contributing.rst now uses a directive to include the contents of CONTRIBUTING.rst. This was tested by building the documentation locally and confirming that the HTML documentation still contained the contributing guidelines.

The result is a streamlined contributing experience from within GitHub, no duplicated content, and no changes to the built documentation.