Separate the Actions jobs into categories

[leouieda]

Instead of running everything under "continuous-integration", separate
into testing, deploying to pypi, and building the docs. This is used in
the other Fatiando projects and seems to work better.

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]

@leouieda the CIs have passed now! If you want I can merge this and update the branch rules to require the new Actions instead of the old ones.