Improve pointers to adding Kiwix (in Raspberry Pi tutorial)

[benjaoming]

The package name kiwix-serve has made people confused. It's currently not obtained as a Debian package (but Kiwix is planning to add it). The way to get it lacks some direct documentation, so the best that we can do is to link here:

https://www.kiwix.org/en/downloads/kiwix-serve/

https://community.learningequality.org/t/not-able-to-install-kiwix-with-kolibri-on-raspberry-pi-3b/1296/1