Add feature flag docs for a11y widget

What does this MR do?

Document the process for enabling the feature flag that is hiding the accessibility merge request widget.

Related to #39425 (closed)

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