Use extlinks for linking GMT docs, and get rid of gmt_module_docs decorator

[seisman]

Description of proposed changes

In this PR, I use extlinks to insert links to GMT documentation, instead of the gmt_module_docs decorator.

:gmt-docs:`coast.html`

will be rendered as https://www.generic-mapping-tools.org/gmt/latest/coast.html.

It's more flexible than the decorator and it can link to any GMT documentations. Thus, we can get rid of the gmt_module_docs decorator.

The only side-effect I know is that help(pygmt.Figure.coast) in interactive mode shows the raw RST string, rather then the link to GMT site.

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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[leouieda]

👍 perfect! I hadn't thought of doing it only on the sphinx side. This is fine and users of the raw docstring don't usually click on many links anyway. Better than having that messy decorator.