12905-c support link reference parsing

What does this MR do?

This adds support for parsing URLs to designs in markdown, parsing them to references. This is part of the work to resolve #12905 (closed), and it also fixes #196736 (closed) by handling the design URLs directly.

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

This adds new computation to the GFM pipeline, so it has the potential to increase markdown processing times. Performance testing suggests that this is negligible but to be safe this functionality is kept behind a feature flag, which needs to be turned on.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

See #12905 (closed)