.gitlab-ci.yml: Avoid running tests in post-merge

CI is currently a huge bottleneck for our project, we can at least improve the situation by running less jobs when those jobs are meaningless.

This patch:

• Avoids running tests in post merge to master or release branches
• Avoids building the documentation only on stable branches
  • We still need to build the documentation in pre-merge, to ensure docs build with every merge
  • We still need to build the documentation in post-merge but only on master, because we publish the docs only from master