[Fix] Documentation build errors

[kohlerca]

This PR fixes some errors during the documentation build process:

1. The docs test job and the ReadTheDocs build job in the CI are based on environment-docs.yml, which specifies python>=3.8. However, the test job sets the environment to Python 3.12, while ReadTheDocs tries to build the documentation on Python 3.14, which is causing errors. The first fix in this PR is to set the Python version in environment-docs.yml to 3.12, ensuring both CI jobs are consistent and the documentation build runs without error.
2. Small errors in docstring formatting that generated Sphinx warnings during the build were fixed.
3. The HTML page generated for the UE tutorial notebook was missing a user-defined function for plotting, due to an explicit request for NbSphinx to hide the cell. It was replaced by the equivalent Viziphant function.

[coveralls]

coverage: 88.303%. remained the same
when pulling fca18cc on INM-6:fix/ci_docs_python_version
into 066c48d on NeuralEnsemble:master.

[mdenker]

Looks all good to me.