docs editing and building instructions not up to date

[janfb]

Our documentation and dependencies still reflect the mkdocs workflow. To fix this we need

• update the doc dependencies, e.g., add all sphinx dependencies
• update the contributing.md on how to edit and build the docs locally
• update the docs README.
• update the publish GH action to automatically build and deploy the Sphinx documentation (it seems sphinx docs are built automatically in the background).