Render reference definitions as code blocks

What does this MR do?

Fixes a bug in the Static Site Editor that removes the blank lines that surround a reference definition. When these blank lines are removed, the reference definitions become broken. For example, the following Markdown snippet, should render something like this:

Other issues just define that, for instance, anything in or to the left
of ~"workflow::ready for development" should be moved as a milestone
comes to a close. A full list of refinement issues is available by
[filtering by the ~"Plan stage refinement" label][refinement-issues].

[Plan project]: https://gitlab.com/gitlab-org/plan
[triage-ops/policies/plan-stage]: https://gitlab.com/gitlab-org/quality/triage-ops/tree/master/policies/plan-stage
[befecheck]: https://gitlab.com/groups/gitlab-org/-/boards/1134263
[groupcheck]: https://gitlab.com/groups/gitlab-org/-/boards/1383681
[refinement-issues]: https://gitlab.com/gitlab-org/plan/issues?label_name%5B%5D=Plan+stage+refinement

### Tracking Committed Work for an Upcoming Release

When the Blank lines around the reference definitions are removed, in other words:

Other issues just define that, for instance, anything in or to the left
of ~"workflow::ready for development" should be moved as a milestone
comes to a close. A full list of refinement issues is available by
[filtering by the ~"Plan stage refinement" label][refinement-issues].
[Plan project]: https://gitlab.com/gitlab-org/plan
[triage-ops/policies/plan-stage]: https://gitlab.com/gitlab-org/quality/triage-ops/tree/master/policies/plan-stage
[befecheck]: https://gitlab.com/groups/gitlab-org/-/boards/1134263
[groupcheck]: https://gitlab.com/groups/gitlab-org/-/boards/1383681
[refinement-issues]: https://gitlab.com/gitlab-org/plan/issues?label_name%5B%5D=Plan+stage+refinement
### Tracking Committed Work for an Upcoming Release

The result is

#243760 (closed)

Screenshots

Before	Ater

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