Look in to alternate markdown shard

[jwoertink]

I found this shard https://codeberg.org/supercell/luce.git

It looks like it doesn't have any C extensions like https://github.com/amauryt/cr-cmark-gfm does. I'd imagine that the speed difference isn't much, but I haven't tested. It might make the website more portable should we decide to move to a different host.