docs: change rational of the gallery grid

[12rambau]

Fix #1121

Choose who goes where

• In the featured area, I decded to keep the earliest adopters of the theme and add some from the maintainers projects (SEPAL, Bokeh, Jupyter Book, MNE) + the most beautiful one (yes I'm thinking about you Arviz).
• All the others goes to the bottom list

Refactoring

I didn't realized that the gallery-grid was used in so many places. I tried to improve compactness, documentation and logic (remove parameters that were legacy and not used).

Docs

The rational is simple: people add their projects to the list at the bottom of the page (clicking on "Edit on Github" will do the trick) and fill the list. Then in a second step, if the maintainers agree that the project deserve biggest highlight (e.g. fancy customization), then we can promote it to the gallery file.

TODO

I'm not very satisfied with the wording in gallery.md feel free to update it.

Side note

I also realized that this directive is only working for MyST. I'm not a big fan of this as it prevents anybody to use it in a normal rst file. I'll open an issue for that later.

https://pydata-sphinx-theme--1390.org.readthedocs.build/en/1390/examples/gallery.html

[drammock]

@12rambau I pushed some commits here:

• fixed check for the option name card-class -> class-card (otherwise setting a custom class didn't work). Note that in the end I didn't set any custom classes but I was trying it out locally.
• small tweaks to docstrings and the text on the gallery page
• made both featured projects and other projects lists alphabetical
• made featured projects cards bigger (so you can see the screenshot better)
• make "other projects" cards smaller
• moved PyVista from feature projects to other projects because its screenshots keep failing (the images aren't shown)

If you strongly disagree with any of this it should be easy enough to revert the commits, I tried to keep the changes separated.

Unfortunately now the CIs are failing everywhere due to nodeJS download errors, try re-triggering the CIs if you get a chance. (also I don't have permissions to re-trigger RTD builds, do you? If not just push an empty commit I guess)

[12rambau]

@drammock thanks for all your modifications, I liked it !
You had missed to remove pyvista from the warning list but it's good now.

All tests are green so I'm going to merge this one !