Document token permissions required by campaigns
Created by: mrnugget
This fixes #12888 (closed) by:
- linking to the code host specific documentation from the campaigns docs
- updating the GitLab documentation to make clear which permissions are required by campaigns
- adding a section to the Bitbucket Server docs to mention which permissions are required in general and by campaigns (some of that is duplicated from the schema, but I think it's much more discoverable up there)
- extending the comment in the Bitbucket Server schema to mention Write access for campaigns