Point to GitLab MR upstream documentation

This is part of the "update the contributors doc" task.

https://redmine.tails.boum.org/code/issues/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

Migrated from https://redmine.tails.boum.org/code/issues/17226.