Add GFM reference format for alerts

What does this MR do?

Adds support for a ^ reference prefix for alerts in GitLab-Flavored Markdown.

The prefix ^ will be recognized in GFM as links to alerts (from Operations > Alerts).

Example usage might look like: ^123. Full URLs will be formatted in the same way for users who have the ability to view alerts on the project (developer+).

Related issue: #220563 (closed)

Screenshots

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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