doc: update collaborator guide
This also updates some parts of the onboarding. It is mainly about
how to handle pull requests, how and when to start a CI, when to add
the ready
label and some other minor changes.
I believe that adopting these suggestions will improve our general PR handling a lot.
It can probably be further polished though, so I am looking forward to your comments.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc