Modernize the front page of the docs

[leouieda]

Use sphinx-panels to make the 4 box callouts to sections of the docs.
Stop reusing the README and instead have a single paragraph explaining
what Verde is. Reuse this paragraph in the README since that one needed
some updating as well. Move the links to other versions of the docs into
it's own page. Rename the API reference to something more informative.
Remove the deprecated test function from the API reference.

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.