Automatically publish docs on release

[tomwhite]

Add a GitHub action to deploy the docs for a new release when the release is made.

The manual instructions are here: https://github.com/pystatgen/sgkit/issues/608#issuecomment-891117795. We might also look at tskit's automation for inspriration: https://github.com/pystatgen/sgkit/issues/669#issuecomment-919118970. And #669 is related (although it is waiting on the release of a version switcher feature).