Add deploy workflow

[waylan]

TODO:

1. Create auth tokens for PyPI and GH Pages.
2. Test this. Maybe do a test with TestPyPI and/or an alpha release???
3. Update Release Process in the Contributing Guide (this replaces steps 8 & 9).

Notes:

I'm using pypa/gh-action-pypi-publish because that action is officially part of PyPA and documented in the PyPA Guide (I didn't even look at other options).

Of the many GitHub Pages deployment actions to choose from I chose peaceiris/actions-gh-pages for a few reasons:

1. As we are using org pages, we don't deploy to the same repo. It was not clear to me how to do that directly with mkdocs gh-deploy. We would need to checkout two repos, and deploy to the one while referencing the working files from the other (see step 9 of the Release Process).
2. This action officially supports and documents using external repos with public/private deploy keys (note: public key gets saved to Python-Markdown/Python-Markdown.github.io and private key to Python-Markdown/markdown). The documentation for any other actions I looked at didn't mention how to do this, even if they appeared to have a config option for it.
3. This action also specifically provides a MkDocs Example in the documentation (among many other static site generators), which suggests continued support.
4. The action was one of the few highly rated ones (893 stars at time of publishing).

[waylan]

The only way to test this is to commit it and then tag a release. I guess testing it will have to wait. Fortunately, the way I edited the request process in the docs we retain the manual method as a fallback.