Replaced links to colormap packages with link to third-party packages list in MPL docs

[1313e]

PR Summary

As discussed in #20121, there are more packages that provide proper colormaps for MPL than is currently shown in the colormaps tutorial in the docs.
Instead of modifying this list every time such a thing is created, I replaced the links to colorcet and palettable with a link to the third-party colormap packages list here, as suggested in #20121.
I will add CMasher to the third-party list myself.
I think we should also add palettable and colorcet there to make sure they are still mentioned.

PS: I am not sure if this counts as a 'new feature', so I marked it as 'N/A' below.

PR Checklist

• [N/A] Has pytest style unit tests (and pytest passes).
• [N/A] Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[1313e]

So, the docs error seems to be unrelated to the page I modified (as that renders completely fine), so I'm not sure what to do here.

[jklymak]

Thanks! Agree that the build issue isn't this PR...