Point to GitLab MR upstream documentation

#15186 made it clear to me that we need at least pointers to the relevant GitLab MR documentation and should not take for granted that everyone is already used to GitHub/GitLab PR/MR workflows and web UIs.

Possible starting points:

• https://salsa.debian.org/help/user/index.md
• https://salsa.debian.org/help/user/project/merge_requests/index.md
• https://salsa.debian.org/help/user/discussions/index.md

Parent Task: #15878

Related issues

• Related to #17172 (closed)

Original created by @intrigeri on 17226 (Redmine)