You can render UML diagrams using Mermaid. For example, this will produce a sequence diagram:
sequenceDiagram
Alice ->> Bob: Hello Bob, how are you?
Bob-->>John: How about you John?
Bob--x Alice: I am good thanks!
Bob-x John: I am good thanks!
Note right of John: Bob thinks a long<br/>long time, so long<br/>that the text does<br/>not fit on a row.
Bob-->Alice: Checking with John...
Alice->John: Yes... John, how are you?
And this will produce a flow chart:
graph LR
A[Square Rect] -- Link text --> B((Circle))
A --> C(Round Rect)
B --> D{Rhombus}
C --> D