Update documentation theme to sphinx-book-theme

[leouieda]

Bump up the pinned version of sphinx and the theme. Copy the
configuration and format from Pooch but no major structural changes to
the docs.

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.

[leouieda]

Heads up @LL-Geo @santisoler @MGomezN this means that you will probably want to remove your conda environments and recreate them with the new dependencies for building the docs.

[leouieda]

Related to fatiando/community#35