Keep separate documentation for different versions

[luator]

The documentation on the GitHub pages is currently based on the master branch. Since cluster_utils is now released on PyPI, I expect more users to be using those releases. For them, it could be annoying if the official documentation is only covering the latest development version, where things might be different than in the stable release.

The proper solution would be to host a separate version of the documentation for each stable release plus one for master.

If I understood correctly, the Sphinx theme we are using already has some support for this. However, I'm not sure about the workflow build pipeline we are currently using.

Open questions:

• [x] What needs to be done for the theme to handle different versions (ideally with some menu to switch).
• [ ] How do we need to change the build pipeline to support this.

[luator]

For the theme, this seems to be pretty straight forward:

1. Docs for different versions should be in separate sub-folders
2. Add a simple JSON file to list the versions and their folders.

See https://jbms.github.io/sphinx-immaterial/customization.html#version-dropdown

Unfortunatley, it doesn't seem to be so trivial to make Sphinx follow that structure. Also, the GitHub action we are using for building, doesn't seem to provide any direct support for this. Maybe it would be best to move to readthedocs.

[luator]

Since it seemed to be rather complicated, I decided to move to Read the Docs instead (https://cluster-utils.readthedocs.io). It adds the overhead of another provider but makes version management much easier.

TODOs left:

• [x] Update links everyhere
• [x] Disable the pages docs and the corresponding workflow