Generate Sphinx docs via Github Actions

[riley-harper]

Instead of manually generating the Sphinx documentation and pushing it to Github, we could dynamically generate it in Github Actions. Then these dynamically generated docs could be pushed from the main branch to hlink.docs.ipums.org.

This prevents situations where we forget to generate the Sphinx docs and also prevents situations where developers use different versions of Sphinx and recommonmark and accidentally cause lots of unimportant changes to the docs.