DOC: Support dynamic filtering by tags for gallery examples with Sphinx-Gallery v0.21.0

[Sphinx-Gallery v0.21.0](https://github.com/sphinx-gallery/sphinx-gallery/releases/tag/v0.21.0) introduces tag support for gallery examples, which allows examples to be tagged and filtered more effectively. See https://sphinx-gallery.github.io/stable/configuration.html#tagging-examples-and-filtering for the usage and https://sphinx-gallery.github.io/stable/auto_examples/index.html?#general-examples for a live demo.

This feature would be very beneficial for the PyGMT gallery, which has grown substantially and can be difficult to navigate, especially for new users. Tag support would allow:

* Better discoverability of examples
* More flexible navigation
* Easier onboarding for new users
* Improved documentation structure without major reorganization

## Potential checklist

- [x] Decide if we want to support tags
- [ ] Decide on the list of allowed tags for PyGMT
- [ ] Document the list of allowed tags
- [x] Upgrade to Sphinx-Gallery ≥ 0.21.0
- [ ] Add tags to gallery examples using the new tagging mechanism [definitely in multiple PRs]
- [ ] See if we should reorganize the structure of the gallery
- [ ] Update contributing guides