Document how to use Docker images on the website

[MOZGIII]

We have docker images now, but they are not mentioned at https://plantuml.com/en/download or anywhere else.

Let's add them to the website so people know they exist!

[jsgv]

For anyone also struggling with usage:

• docker run -v $(pwd):/data plantuml/plantuml "directory1/diagram.puml"
• docker run -v $(pwd):/data plantuml/plantuml "-stdrpt:1" "directory1" "directory2"