Rename "master" to "main" throughout the project

[leouieda]

Update the default branch name is CI and testing. The default in the
Pooch class remains "master" for backwards compatibility and won't be
changed until v2.0. Updated the links to our guides as well.

Fixes #276

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]

Looks great, hope we don't break any package that depends on Pooch 🤞🏼.

I've just replaced "master" for "main" on CONTRIBUTING.md and MAINTENANCE.md.
We are still defaulting to "master" on pooch.create(), which is good so we keep backwards compatibility. We should deprecate that in the future and change the default to "main".