Add tag-based filtering

[pradyunsg]

This would allow users to filter through the long list of themes based on some characteristic of the themes.

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Here's my starting list for potential tags we can have.

Layout:

• "two-column"
• "three-column"
• "centered"
• "responsive"

Number of knobs:

• "can-change-colors"
• "can-change-content-width"
• "can-change-layout"

[pradyunsg]

Coming from #3, it might be an idea to have a separate section for themes that are "derivative" or associated with specific projects.

[mgeier]

Given how long it takes to simply port the themes over to the new schema (see #25), it will be basically impossible to ever get those tags correctly up to date, if done manually.

The only way this could work, is if the tags are somehow auto-generated.

It would probably be possible to detect media queries (for screen size and dark mode) in CSS and viewport and similar things in HTML, but I don't know whether this would be reliable.

Regarding "derived" themes: I would simply kick out all trivial derivatives from the list and only keep ones which add a significant contribution (or if the original theme is unmaintained).

The "associated-with-some-project" should be irrelevant (unless it is only allowed to be used with that project, in which case it should be kicked out of the list). If people are looking for the theme of some project, they can simply search for the name on the page. If the project name is not part of the PyPI name, it can easily be added to the "description" part.

Apart from "tags", I think it would be great to have some sorting options. By default it's alphabetical by PyPI project name, but it could also be interesting to sort it by actual theme name (which might or might not be different).
It would also be interesting to be able to sort by latest PyPI release (would this be doable?).
If PyPI data is available, there might be other options to sort by?

[pradyunsg]

With the trimming of themes that took place in #72 and #73, this is now significantly more feasible. Nearly all the themes that were trimmed were derivatives of/org-specific forks of sphinx-rtd-theme anyway.

"derived-from-readthedocs"

This is no longer necessary. The bar is now "be significantly different to get in".

"associated-with-some-project"

This is also no longer necessary, since project-specific themes are not usually meant for reuse by other documentation -- which is what the site is now refocused toward.

[pradyunsg]

A PR for doing this would be welcome! ^>^