Check for broken references in the documentation

[saraedum]

This makes the CI fail when there are invalid references in the documentation. (I added a lot of those in #158.)

Checklist

• [x] Added an entry in doc/news/.
• [x] Added a test for this change.
• [x] Added new .py files to the documentation in doc/geometry or doc/graphical. Nothing new.

[saraedum]

This is a bunch of rather bizarre ones @videlec:

/usr/share/miniconda3/envs/test/lib/python3.10/site-packages/flatsurf/geometry/finitely_generated_matrix_group.py:docstring of flatsurf.geometry.finitely_generated_matrix_group:2:py:class reference target not found: EmptySetError

If you grep for EmptySetError it's nowhere to be found. Apparently, we now inherit doc strings in sphinx and EmptySetError is in Parent.an_element, see https://flatsurf.github.io/sage-flatsurf/geometry/finitely_generated_matrix_group.html#flatsurf.geometry.finitely_generated_matrix_group.FinitelyGenerated2x2MatrixGroup.an_element.

So we need to add a few docstrings to our code to make these warnings go away.

[github-actions[bot]]

Documentation preview for this PR is ready! :tada: Built with commit: 223121b61f6274e35e78254d10724fbfc56db5cf