Add tutorial on how to build with distrobox
This requires !435 (merged). Do not merge this MR before that one!
This MR ports and rewrites the distrobox portion of https://community.kde.org/Get_Involved/development/More#Develop_in_a_Linux_container
In addition to being more succinct and more "instruction-y", it also mentions how to change an openSUSE Tumbleweed container into a Krypton one so as to have an option aside from KDE neon Unstable.
It also makes it so we don't recommend only Plasma-from-git containers, since with kdesrc-build a sufficiently up-to-date distribution is okay.