Reconfigure Builds and perform more doctests

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Splits the notebooks recipe in the tox.ini file
• Sets the GitHub Workflows to test the doctests across more builds
• Stages the tox-gh plugin for eventual integration

Does this PR introduce a breaking change?

Yes, tox-gh is now a development dependency and the "notebooks_doctests" tox recipe has been replaced with "notebooks", while doctest has been made into a modifier for typical tox runs to also run the documentation examples.

Other information:

https://github.com/tox-dev/tox-gh

[github-actions]

Note
It appears that this Pull Request modifies the main.yml workflow.

On inspection, the XCLIM_TESTDATA_BRANCH environment variable is set to the most recent tag (v2023.12.14).

No further action is required.

[coveralls]

coverage: 90.638% (+0.4%) from 90.19%
when pulling 41fb7a8 on split-notebooks
into 2d8726b on main.

[aulemahal]

Looks good!