Fix issues with PDF docs generation

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• HISTORY.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?

• Prevents PDF builds of the documentation (typically run after the linkcheck and html recipes have been built) from running the notebooks

Does this PR introduce a breaking change?

Yes. The notebooks will no longer be run when either of the following are true:

• if env SKIP_NOTEBOOKS=1
• if env "READTHEDOCS_VERSION_NAME" in ["latest", "stable"] or env "READTHEDOCS_VERSION_TYPE" == "tag"
  • and additionally, if env("READTHEDOCS_OUTPUT") in ["pdf"]

Other information:

https://docs.readthedocs.io/en/stable/reference/environment-variables.html