Link to PlantUML details in Markdown page

What does this MR do?

Adds a link in the Markdown page to PlantUML documentation already present in the Administrators section. Here I've decided to keep the instructions along with the configuration steps.

We might also consider moving the instructions to the Markdown page.

Related: #35427 (closed)

Screenshots

N/A

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

N/A

Security

N/A