Migrate Gallery to Catalog

[pawamoy]

Migration of the code in https://github.com/pawamoy/mkdocs-gallery.
The code now use the local projects.yaml file instead of downloading it from GitHub.
@oprypin listed as co-author.

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TODO once merged and deployed: update the generator to use URLs from mkdocs.github.io/catalog for theme thumbnails

[pawamoy]

Careful review needed here: I'm not sure deploying to GitHub pages is desirable, given MkDocs documentation is served under its own custom domain.

[Andre601]

Deploying it like that would be the same as pushing to a gh-pages branch here, meaning it would be under https://mkdocs.org/catalog, assuming the deployment happens on this repository, which is what I assume.

[pawamoy]

Thanks, that's what I'm suspecting too. @tomchristie can you confirm that docs pushed on the gh-pages branch of mkdocs/catalog will end up at www.mkdocs.org/catalog (not that I want to proceed, just want to understand how it works)?

[Andre601]

A small thing to consider, but should themes offering multiple variants display all their variants?
By "variant" I mean things such as a light and dark theme.

It could help showing a user the possible options there are. And right now are there at least two themes with such schemes available (Material for MkDocs and the MkDocs theme)

[pawamoy]

Interesting point, yes, why not. That would play well with the hosting site which can itself be used in both variants (showing light variants or dark variants depending on the current theme selected by the user browsing the site). Maybe not that useful to display both variants next to each other though (takes more space and we don't have a lot, at least in the catalog, and users can reach the demo and see the variants there for themselves, interactively).