Add PlantUML details to markdown page

Problem to solve

Currently, our markdown documentation mentions Mermaid JS and not PlantUML - both of which are now supported in GitLab flavored Markdown. See: https://docs.gitlab.com/ee/user/markdown.html

Further details

• For PlantUML, additional work is required of GitLab administrators to make it work
• This work is documented here: https://docs.gitlab.com/ee/administration/integration/plantuml.html#plantuml--gitlab
• That documentation page then does have some syntax around adding in PlatUML to GitLab

Proposal

Either move the markdown syntax items to the Markdown documentation page, or at least link to it. I see this similar to Pages in the sense that we document its use for users and the required work for administrators differently.

And in fact it is enabled on GitLab.com so users should be encouraged to use it there.