Add filtering tags for gallery examples of projections

[seisman]

Changes in this PR:

• Bump to sphinx-gallery >= 0.21.0
• Add tags for projection gallery examples [Based on the figure and table in https://docs.generic-mapping-tools.org/dev/reference/options.html#coordinate-transformations-and-map-projections-the-j-option]

Preview: https://pygmt-dev--4643.org.readthedocs.build/en/4643/projections/index.html

The first PR to address #4599

[yvonnefroehlich]

Looking at the example by Sphinx at https://sphinx-gallery.github.io/stable/auto_examples/index.html?#general-examples, there should be a nice tag symbol. However, in the preview of this PR, it looks a bit strange. Besides this, I feel it could improve the structure if "Tags" were written in bold font, but I think we cannot change this.

Preview of this PR	Example by Sphinx

[seisman]

there should be a nice tag symbol. However, in the preview of this PR, it looks a bit strange.

I can see the tag symbol in the preview, so it's likely a browser issue.

I feel it could improve the structure if "Tags" were written in bold font, but I think we cannot change this.

Actually we can, I've adjusted the style in commit fe40f73

[yvonnefroehlich]

there should be a nice tag symbol. However, in the preview of this PR, it looks a bit strange.

I can see the tag symbol in the preview, so it's likely a browser issue.

Your are probably right, on my smarphone it looks good.

I feel it could improve the structure if "Tags" were written in bold font, but I think we cannot change this.

Actually we can, I've adjusted the style in commit fe40f73

That's good and with the changes it looks much better.

[weiji14]

I think only Mollweide (equal-area) is missing a tag. Otherwise should be ok.

[seisman]

I think only Mollweide (equal-area) is missing a tag. Otherwise should be ok.

Nice catch. Fixed in f71ee74.