ci: add trigger job and only run documentation job post-merge

• require manual trigger to run pipeline on branches and MRs
• require manual trigger to run pipeline post-merge (excl. docs)

Main goal is to reduce runner workload of gst-plugins-rs pipelines which currently run all the time and thus cause way too much load on the CI infrastructure. See gstreamer-rs#417 (closed)

Future improvements:

• maybe always run lint stage, or add separate trigger for lint stage (depends on peoples' workflows really, do they really rely on the ci job for rustfmt instead of just doing it locally?)