PlantUML support for Markdown

What does this MR do?

Add support for rendering PlantUML diagrams in Markdown documents.

Are there points in the code the reviewer needs to double check?

This MR is a different implementation of MR !8553 (closed). This implementation uses HTML pipeline filter to generate the PlantUML diagrams while MR !8553 (closed) extends Redcarpet gem to do the rendering.

Why was this MR needed?

PlantUML is already supported in Asciidoc so why not Markdown too...

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

MR !8553 (closed) Issue #4048 (closed)