sdn: overall language and consistency rework

sdn: overall language and consistency rework

This is a general style polishing rework that wants to:
- improve consistency of spellings and casing
- drop some to verbose examples that included the whole generated
  ifupdown2 config, which is not only not that useful here, but will
  also get outdated fast, so it is probably more confusing than
  helping at the current development pace. Could be nice in some
  reduced form some times in a "inner workings" section
- shorten some examples verbosity by not spelling repetitions of
  steps, e.g., for those that one needs to apply to all guests in an
  example.

Best to check the changes using the following git flags to avoid noise:
git show -w --word-diff=color --word-diff-regex='\w+'

Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
 [ TL: fix build, squash patches to not break patch causality, add
   actual commit message and reinstate some notes to avoid loss of
   information ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>