Replace cartopy with pygmt in docs

[santisoler]

Description of the suggestion:

Would be great if the maps in the User Guide are done through PyGMT instead of Cartopy. To do so we need to change how the maps are done and include pygmt and gmt as dependencies both in environment.yml and in envs/requirements-docs.txt.

[mdtanker]

I'm happy to do this! Let me know and I'll start working on it.

[santisoler]

That would be awesome @mdtanker ! Feel free to start whenever you want. I'll assign you to this issue so anyone else interested knows that you're working on it.

[mdtanker]

Okay great, I'll start converting them. I've just finished converting the 6 User Guide - Sample Data examples, and will start working on the Gallery examples.