Add mini-galleries for methods and functions

[seisman]

Description of proposed changes

This PR updates the autosummary templates, to show mini-galleries of all methods and functions at the end of the documentation page.

Here is the screenshot for the documentation of the pygmt.Figure.plot() method:

It's better to see the differences by comparing the API documentation of the current dev version and the preview of this branch.

• Current dev documentation: https://www.pygmt.org/dev/api/index.html
• Preview of current branch: https://pygmt-git-method-gallary.gmt.vercel.app/api/index.html

References:

• https://sphinx-gallery.github.io/stable/configuration.html#add-intersphinx-links-to-your-examples

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.

[weiji14]

This is brilliant! Thanks @seisman!