Add GitHub action workflow to build & publish doc

[n-peugnet]

• Fix include directive error message:

  doc/index.rst:1: CRITICAL: Problems with "include" directive path: InputError: [Errno 2] No such file or directory: '../README'.

  This error may have appeared when upgrading the maximum Sphinx version.

• Add GitHub action workflow to build & publish doc

You will need to follow the steps (1) to (4) from this GitHub doc, then once merged the doc should be accessible at this URL: https://markstory.github.io/sphinxcontrib-phpdomain/

Fixes #45

[markstory]

Thank you 🎉