CI: re-use installed package for building pages / drop sphinx-autodoc

[elcorto]

We install all deps and the package for tests for each Python version (well, ok) but then again once more for building pages, because we use https://github.com/elcorto/sphinx-autodoc which needs an installed package. Either find a way to re-use a previous install or drop sphinx-autodoc for a more standard sphinx workflow where we (probably?) only need to parse the package source w/o installing it.