GitHub Pages replacement for ReadTheDocs

[jpmckinney]

We might prefer this in order to:

• Eliminate a third-party dependency (though adding a new workflow to maintain)
• Avoid ads on our documentation (though we could pay a monthly fee to remove them)

ReadTheDocs is also sometimes fussy and hard to debug.

Spoonbill will build its docs to GitHub Pages, so we can copy that workflow when ready.

Sphinx has a sphinx.ext.githubpages extension: https://www.sphinx-doc.org/en/master/usage/extensions/githubpages.html

[jpmckinney]

See also #35

[jpmckinney]

ReadTheDocs has been making improvements (its configuration file makes builds much more predictable) and it has many unique features (like redirects), so we will continue to use it, rather than try to re-do things on GitHub Pages.