CI pipelines for regenerating and deploying documentation website via hugo.

[bpkroth]

We should revisit hooking up documentation generation via hugo to the CI pipelines at some point.

Also documenting how to do it even if it's still manual.

Originally posted by @bpkroth in #14 (comment)

[amueller]

I'll write proper documentation on this later, but for now the process is:

Install hugo and build the hugo part of the site:

sudo apt-get install hugo
cd website
./build_site.sh
rm -rf public/*
hugo

install sphinx and build the sphinx part of the site:

cd sphinx
pip install sphinx sphinx-rtd-theme numpydoc
make

Copy the sphinx part into the hugo part:

cp -r _build/html/ ../public/python_api

The easiest way to push the website that now lives in website/public to github is with

 git worktree add -B gh-pages website/public upstream/gh-pages

If a new class was added, you also need to run the sphinx source generation with

./apidoc.sh

and then reset some files with

git checkout .

easy-peasy!

[bpkroth]

@amueller this got done in #50 right? I guess #55 is related as well.

[bpkroth]

This got started in #50. #61 applies some additional fixups and extends it to include link checking as well.