Update documentation for Reviewing, Merging and Declining Pull Requests

Description

The documentation on https://docs.opencast.org/develop/developer/reviewing-and-merging/ does not reflect some recently passed proposals (no merge ticket anymore, no merge order on features anymore, not translation key changes in release branches).

Also, add some module-specific guidelines

Fixed and reviewed

Assignee

Sven Stauber

Reporter

Sven Stauber