Document Mattermost support process

In https://gitlab.com/gitlab-org/gitlab-ce/issues/39703 we talked with the Mattermost team about the future of bundling and support. The Mattermost team is going to assist GitLab closely to continue shipping MM with GitLab.

With that in mind, the Mattermost team have written up the support process at https://docs.mattermost.com/process/gitlab-process.html#service-level-agreement-sla.

We should write in the support handbook the method for contacting their support team (which is via the issue trackers with a mention to their support group).