Markdown syntax is supported through the recommonmark library.
PlantUML syntax is however not directly supported, so it's necessary to use something like the plantuml service to render diagrams. For example:
@startuml
Alice -> Joe: Authentication Request
Joe --> Alice: Authentication Response
Alice -> Bob: Another authentication Request
Alice <-- Bob: another authentication Response
@enduml
Equates to the URL: http://www.plantuml.com/plantuml/uml/TSx12O0m38NXUwV80bb0HAf7ZsvGQy229chJ_GqgUD7p-_XvFciujAoBQvSK2R21NgY2jkXCh2a6JS8mq5uegxCDqCo5FrJUX3EvDzR9Q8p5t07XEthZ6l7MuKy_SKyysU4J
The encoding of the UML text to a URL path element is interesting. An example of an encoder in python: plantuml.py