Add docs for GFM reference filter recommended format

What does this MR do?

In introducing new GitLab-Flavored Markdown (GFM) reference formats for iterations and alerts, we've landed on what should be a default format for new GFM prefixes.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done