[DOC] Rework API doc of variables module

What does the code in the MR do ?

This MR proposes to clean and improve the docstrings of the leaps.variables module.

Where should the reviewer start ?

The reviewer can take a look at the rendered documentation: https://leaspy--189.org.readthedocs.build/en/189/reference/api/leaspy/variables/index.html

How can the code be tested ?

By reading and trying the auto-generated links of the rendered version.

When is the MR due for? (review deadline)

What issues are linked to the MR ?