Add sphinx doc builds to master, push to Github Pages for this repo

[dotsdl]

We want to start building out docs for the components exposed by fah-xchem as we develop them further. In the past we would use ReadTheDocs for this, but conda installs there are slow and prone to timeouts (or memory exhaustion), so we may be better served by sticking with Github Actions for this.

We could write an Action that builds the docs and pushes them to the gh-pages branch of the repo. This would work fine for docs built from e.g. master, but may not work well if we want to support display of docs for many different releases. We can proceed with docs built only for master for now.