added GH action for generating docs on release

[erickmartins]

Description

This PR adds a github action for generating documentation on release using Sphinx and force-pushing it to the documentation branch. It will require a minor change in repository settings (from deploying github pages from the docs directory to root directory in the documentation branch). It also allows this action to be run on-demand when a documentation update is necessary.

Checklist

• Docstrings for all functions
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.

[govekk]

Looks good to me!