Improve Sphinx's usability with GitHub Pages

[AA-Turner]

This is a summary of the actionable content from #3382

Action 1: We should add a target that renders documentation and then commits it to the gh-pages branch, if currently within a git repository. This would be similar to MkDocs' gh-deploy option. (see #3382 (comment))

Action 2: We should also add a brief guide (perhaps in sphinx.ext.githubpages) on how best to configure sphinx-build et al for deploying to GHP, if one wants to commit rendered documentation to the master branch. (Similar to #3382 (comment))

A

[pradyunsg]

Is there any reason to not be generating the nojekyll file by default?