Integrate theme gallery on www.mkdocs.org

[pawamoy]

Some time ago I created a very simple gallery of themes: https://github.com/pawamoy/mkdocs-gallery.
It was then integrated in the catalog, by adding links to the deployed gallery at https://pawamoy.github.io/mkdocs-gallery/.
The goal is now:

1. to fully merge the gallery code into the catalog repository, and there's a PR for that: Migrate Gallery to Catalog catalog#165
2. somehow integrate the gallery on www.mkdocs.org

For point 2, I can see a few options:

• we just publish the gallery at www.mkdocs.org/gallery, like the one at https://pawamoy.github.io/mkdocs-gallery/ but obviously using the mkdocs theme (currently in use at www.mkdocs.org) instead of the material one. Each theme preview is either served at www.mkdocs.org/gallery/theme/X, or www.mkdocs.org/gallery/X, depending on whether we'll want to have other galleries (plugin gallery?). I'd argue that each plugin should be previewable with every theme, so I'm leaning onto www.mkdocs.org/gallery/X. One limitation is that the home page of the gallery is not integrated within www.mkdocs.org: it's only a subsite which doesn't share the same navbar, CSS, etc.
• we tightly integrate it into a specific page of www.mkdocs.org. Since the gallery is auto-generated from the catalog, we'd have to add the code responsible for generating it into MkDocs' own docs build process.

I'm not sure option 1 is actually possible, because if we use mkdocs gh-deploy from the catalog, it would create pages at www.mkdocs.org/catalog, not gallery, or mkdocs.github.io/catalog if the custom domain doesn't apply to subsites. Maybe we should keep the catalog and gallery repositories separate, but that would mean more difficult auto-update of the gallery when the catalog changes.