Refactor Catalogs

[savente93]

Issue addressed

Fixes #844

Explanation

Another attempt at merging #849. See that PR for more information, but I'll detail the changes I've also made here:

1. I've updated the README in the catalogs a bit more, including how testing can and should be performed.
2. The registry files were taken from the github.com domain instead of raw.githubusercontent.com domain meaning that HTML files were retrieved instead of the registry files themselves causing errors in pooch.
3. I've added some minor changes to the examples to get the docs working again.
4. Since the process of adding/modifying the predefined catalogs is a bit more complicated I've also added a separate checklist of easy mistakes to the PR template.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Tests & pre-commit hooks pass
• Updated documentation if needed
• Updated changelog.rst if needed

Additional Notes (optional)

As we saw during the last time this got merged, it is really difficult to see what the docs will do once it's merged. As such, I don't think there's much we can do now, but just get the PR through review, merge it again, and then deal with any issues that come up then. I'll be available to diagnose and fix any issues once this gets merged.

[Tjalling-dejong]

only some typos, other than that LGTM

[Tjalling-dejong]

LGTM