live-task-localisation: include Debian Reference for all languages

debian-reference recommends packages for all the languages. Previously, we were only installing the Italian & Spanish packages.

debian-reference provides PDF, HTML, and text versions. The text version is gzipped so one way to read it is with

zcat /usr/share/doc/debian-reference/docs/debian-reference.en.txt.gz | less

live-task-localization is included in the Debian Live Standard ISO which is a command line version of Debian. If desktop maintainers don't want debian-reference at all, they should just add Conflicts: debian-reference-common to their task-live package.

Closes: https://bugs.debian.org/1110658

cc/ @jcc @rclobus