Combine Ubuntu and Debian instructions
What does this Merge Request do?
Debian and Ubuntu instructions can be the same, so let's combine them and call out the few differences.
CC @avar
Merge Request checklist
-
Tests added for new functionality. If not, please raise Issue to follow-up. -
This change is backward compatible. If not, please include steps to communicate to our users. -
Documentation added/updated, if needed. -
gdk doctortest added, if needed.