Minor improvements to the docs

[santisoler]

Fix reference to DatasetAccessorPrismLayer on prism_layer docstring. Replace
Equivalent Layer for Equivalent Sources on API Index and on links to their
methods in docstrings. Fix links to datasets.fetch_... functions in synthetic
surveys docstrings and shorten them. Remove additional backticks on :func:
directive for numpy.meshgrid in prism_layer.

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.