Update documentation to use pdoc and refresh automatically using github action.

[hhoppe]

The current documentation in https://google.github.io/mediapy/ was created using pdoc3 which was a fork of pdoc. The documentation was updated manually (and infrequently) by writing to a gh-pages branch in the repo --- a cumbersome process.

The maintainers of pdoc have been very active and so it is now worthwhile to transition to that package.
See the discussion at the end of https://github.com/mitmproxy/pdoc .

The new mediapy documentation can be previewed at https://hhoppe.github.io/mediapy/
(That view is still not showing some logo images because these are awaiting their commits into the main https://github.com/google/mediapy/ repo.)

One issue with pdoc (and pdoc3) is that they expand all type information -- see mitmproxy/pdoc#420 . This PR avoids the problem by introducing temporary typenames and applying some postprocessing. Hopefully this can be simplified in the future.

To enable web deployment of the new documentation, it is necessary to change the GitHub repo setting Pages to use as source GitHub Actions rather than Deploy from a branch: gh-pages.
Perhaps it is best to make this settings change before applying the PR to enable the website github action to operate correctly. And/or this may require an additional dummy commit.

Once the new documentation is live, it will be possible to delete the gh-pages branch as that will no longer be needed.