Improvements to docstrings of Ellipsoid and Sphere

[leouieda]

Standardize the docstrings of Sphere and Ellipsoid, add (and fix) missing reference to
Mortiz H. for the mean radius equation, link docstrings of both classes, improve the doc
tests for Sphere.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

Now docstrings look way nicer! Thanks!