Add GFM reference format for feature flags [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR does the following things:

• Support the reference for Feature Flag objects in markdown.
• Users can mention [feature_flag:<IID>] in their notes to refer a specific feature flag. The generated link is to jump to the flag edit page.
• The cross-project reference is supported e.g. [feature_flag:gitlab-org/gitlab/1].
• Document the capability.

NOTE: This is NOT Feature flags in development of GitLab.

Related #220333 (closed)

Screenshots (strongly suggested)

This is the screenshot to add a reference in a note and jump to the corresponding flag edit page.

Authorized users see an active link and a tooltip for the flag name. (Feature Flags can be read by Developer or above)

Unauthorized users see an inactive link and cannot read the flag name.

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team