Create ZIM file for Kiwix in build process

It would be neat to have a ZIM file (for use in Kiwix and other similar readers) for offline access to the documentation.
A thread on discord showed that it's possible to create it locally or in this case on the build server.

The command that creates this is as follows:
```sh
zimwriterfs \
-w index.html \
-I zimfile-illust.png \
-l eng \
-n "gbdev-pandocs" \
-t "Game Boy™ Pan Docs" \
-d "Community-created Game Boy™ development documentation" \
-c "GBDev Community" \
-p "GBDev Community" \
-a "gameboy;game boy;nintendo;development;assembly;programming;dev;asm;gbdev;pandocs" \
-e "https://gbdev.io/pandocs/" \
BUILD-OUTPUT-DIR/ \
pandocs.zim
```

The flags here are
* `w`: default page
* `I`: icon for the zim file (this can be created on the fly with imagemagick or just added to git)
* `l`: language code (3 letters)
* `n`: name of the file (used interally for stuff)
* `t`: user-facing title
* `d`: description
* `c`: creator (of the content)
* `p`: publisher (creator of the zim file)
* `a`: arbitrary tags
* `e`: original source URL

Would be cool if we can get this step added to the build process so the ZIM file is created automatically, and it could be added to the github binaries list, or of course linked elsewhere too, perhaps even from within pandocs itself.