Add more information and refactor format of branch-related API topics
What does this MR do?
Establishes closer links between the branches API documentation and other part of the doc suite. Also does some reformatting for consistency.
Related issues
Relates to: https://gitlab.com/gitlab-org/gitlab-ce/issues/10421.
Author's checklist
Review checklist
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Your team's review (required) -
Technical writer's review (required)
Edited by Evan Read