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.

---

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.

[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)?

[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).

[pawamoy]

Closing, feel free to take it from there.