Add mercury and moon spheres to realizations.py

[dabiged]

Added Mercury and Moon Spheroids, plus documentation. This completes #53 .

Reminders:

• [x] Run make format and make check to make sure the code follows the style guide.
• [x] Add tests for new features or tests that would have caught the bug that you're fixing.
• [x] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [x] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [x] 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.

[leouieda]

@dabiged thanks for this! I made a few minor fixes to the docs formatting. The pages were missing from doc/ellipsoids/index.rst and I forgot about it in the previous PR as well. When CIs finish, I'd be happy to merge this.