Remove pbuilder workflow, add git and sbuild usage and fix grammar

See merge request !229

* CONTRIBUTING.md: Make sure the guide explains what the commands
  do, how to progress after cloning this repository, deprecating
  the pbuilder workflow in favor of clarifying what sbuild expects
  and does, as well as how to test the builds in isolated sessions
  and how to use git to do a proper commit instead of jumping to a
  push.

backport from eoan

See merge request !221