Version switcher

Version switcher is controlled by docs/sphinx/_static/switcher.json file. When releasing a new version, remember to add it there.

The path to the version switcher in conf.py is relative to the webserver root, so it should work both locally with inv docs-view (see how tasks.py are tweaked to serve from /cuda/nvmath-python) and on docs.nvidia.com (our docs are at /cuda/nvmath-python there as well).