Optimize GitLab CI configuration (#8499)

* tests are not executed for doc-only changes
* images will be (once merged) downloaded from GitHub Registry so likely much faster
* we have a "scheduled" nightly build that will build everything from scratch and check if no requirements have been broken
* improved split of static checks between two static check jobs - to utilise parallelism better.
* reorganised some fast jobs (requirements, prod image) that do not depend on tests so that they can run earlier
* shorter names for jobs so that they are nicer to view in the actions view
* matrix definitions of the jobs so that we can manage them better