Mention salsa.debian.org and Salsa CI

Otto Kekäläinenrequested to merge
otto/debmake-doc:salsa-and-salsa-ci-basic-mentions into master

While salsa.debian.org was visible in some command examples, there was no actual section about Salsa nor Salsa CI. Add some basic mentions to cover them. A longer explanation can be added later.

(Updates guide at https://www.debian.org/doc/manuals/debmake-doc/index.en.html)

Edited by Otto Kekäläinen

Merge request reports