I'd like to be able to install one particular package (and keep it updated) from Arch: arch-wiki-docs (also here).
I know it is not recommended to to this, but it's basically just data so it should not hurt.
Suppose it can't be done, how can I go about to unpack the downloadable .zst file on Ubuntu?
In this related question virtually everybody seemed to be opposed to the idea of doing it. So I guess the answer is: dont't.
However, to unpack the .zst file all you need is to install zstd:
apt install zstd
... and unpack it with tar -xf file.tar.zst.
In case of the arch-wiki-docs package, the contents will end up in usr/share/doc/arch-wiki/html (relative to your current folder, unless you run as root and cd /).