Add complete listing of categories to packages page

[palmskog]

The discoverability of packages on https://rocq-prover.org/packages is currently very low - users are basically forced to search by entering a text term to find anything at all.

To aid discoverability, I think a complete listing of (clickable) package categories should be added to https://rocq-prover.org/packages - this is something even the old website had.

[palmskog]

Apparently #5 has mention of categories as well, but this issue is specifically about giving a full list of categories for discoverability.

[spitters]

I'm not entirely sure what discoverability you are after, but this may help a bit.
https://sherlorocq.sirref.org/

Although, it looks like some dev/testing packages have been included, which may not be what we want, and some packages seem to be missing. I haven't had the time to investigate yet.

[palmskog]

@spitters I mean discoverability in the sense of how easy it is to figure out "what nice packages are available" related to some general topic such as automata, program semantics, probability theory, etc.

Unfortunately, neither sherlorocq or https://rocq-prover.org/packages provide answers to this, but rather serve users who already know specific keywords. Of course, we can still link to sherlorocq on the website as a complement to ordinary package search.

[CohenCyril]

I'm not entirely sure what discoverability you are after, but this may help a bit.
https://sherlorocq.sirref.org/

Oh my, that's impressively fast!!!! I wonder if we can do the same adding in semantic searchability.