Refactor broadcast message API page
What does this MR do?
Standardises structure and content in broadcast messages API page. Also establishes links to main concept material.
Related issues
Relates to: https://gitlab.com/gitlab-org/gitlab-ce/issues/10421
Author's checklist
Review checklist
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PM's review (recommended, but not a blocker) -
Technical writer's review (required)
Edited by Achilleas Pipinellis