Do not build EPUB on ReadTheDocs

[Zeitsperre]

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes issue #xyz

• [ ] Tests for the changes have been added (for bug fixes / features)

• [x] Documentation has been added / updated (for bug fixes / features)

• [ ] HISTORY.rst has been updated (with summary of main changes)

• [ ] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:

• Drop EPUB support for CLISOPS on ReadTheDocs

• Does this PR introduce a breaking change?:

No.

• Other information:

EPUB file does not fully support Jupyter Notebooks, so building them causes fail-producing warnings.