Clean up doc publication workflow

[leofang]

I'd also like to use this opportunity to purge the doc artifacts from the main/release branches, following the common practice. In short:

• The main/release branches should only contain the doc sources (= source code + Sphinx rst files)
• The gh-pages branch that we use to publish the docs should only contain the doc artifacts (=html files)

Originally posted by @leofang in #95