Use the minigallery directive to insert mini gallery in API documentation

[seisman]

Description of proposed changes

Currently, the mini-gallery at the end of each API documentation is added using the following include directive:

.. include:: backreferences/{{ fullname }}.examples

This include directive was originally added to the class template in PR #383 (Nov, 2019) since there are no other way to insert mini galleries at that time. The same include directive was then added to the method and function template in PR #648 (Oct, 2020).

Since v0.7.0 (PR sphinx-gallery/sphinx-gallery#685, released in May, 2020), Sphinx-Gallery provided the minigallery directive (https://sphinx-gallery.github.io/stable/configuration.html#add-mini-galleries-for-api-documentation) to add mini gallery.

This PR replaces the outdated include directive with the better minigallery directive as suggested by the official sphinx-gallery documentation (https://sphinx-gallery.github.io/stable/configuration.html#add-mini-galleries-for-api-documentation).

Preview: https://pygmt-dev--2459.org.readthedocs.build/en/2459/api/index.html

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

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specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version