One should especially focus on the Sphinx update (from v5 to v7), rdflib
update (from v6 to v7) and the necessity to keep pydata-sphinx-theme at
its current version due to too many differences in the look and feel of
the docs if updated to the latest version.

Also, all currently unversioned dependencies have been fixed to a
version.

Avoid using the - quite outdated - ammaraskar/sphinx-action GitHub
Action. It was forcefully using Python 3.8 due to using a very old
version of the underlying docker image sphinx-doc/sphinx.

Support installing all requirements files at once (resolving that there
are no sub-dependency issues).

All warnings have been removed by:
- Move `!pip install` statements to a separate cell in all notebooks,
  making the cell "shellscript" code and add a sentence immediately
  before stating the same thing.
- Add all files to a Sphinx `toctree` tag, specifically extending the
  header nav with the "More" dropdown, which includes "contribute",
  "tools", and "resources".
- Find online and add the image for the Protégé configuration.

Update the configuration to remove the primary sidebar from pages that
have no section navigation.

Since no warnings are produced (locally) now, the CI workflow has been
updated to include the `-W` flag for sphinx-build, which treats warnings
as errors during the build.