Follow-up from "Move Mattermost doc from Omnibus to main project"

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doc/integration/mattermost/index.md has/will come over from https://gitlab.com/gitlab-org/omnibus-gitlab stamps as Enablement/Distribution.

The following discussion from !70516 (merged) should be addressed:

• @bprescott_ started a discussion:

  Looking at https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/user/project/integrations/mattermost_slash_commands.md#L2, wondering if it should be:

  stage: Ecosystem
  group: Integrations

  We should determine which team actually owns this file.