[Debian-salsa-ci] Salsa CI documentation improvements

[Otto Kekäläinen]

> > Very *early* draft at
> > https://salsa.debian.org/salsa-ci-team/pipeline/-/merge_requests/519

Related I was reading the commit message and contents in ab8f0796 I
don't understand it.

Why is

  - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/recipes/debian.yml

better than the widely used existing way?

  - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/salsa-ci.yml
  - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/pipeline-jobs.yml