Add docs for frontmatter delimiters
What does this MR do?
Add missing documentation for https://gitlab.com/gitlab-org/gitlab-ce/issues/52007. Functionality added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23331.
Related issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/52007
Author's checklist
Review checklist
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Technical writer's review (required)
Edited by Evan Read