New automated docs build needed

[nkeim]

The last task running on Travis CI (since #640) was the docs build for the master branch. Since travis-ci.org is now defunct, we need to migrate this to GitHub actions (or similar) if we want up-to-date dev docs. In the meantime, we can build the stable docs manually as part of the release process.

[caspervdw]

Thanks for raising this issue. I have had good experience with readthedocs, they offer easy integration with github. No CI needed. That would mean a new domain for the docs though.