dhall-docs: Syntax highlighting for examples and source

[darichey]

I think this would be a nice addition to the docs tool, but I wanted to open this issue and get some feedback before working on it more.

My initial experiments with prismjs yielded fairly promising results:

Would this be a welcome change?

[Gabriella439]

Yeah, definitely! We originally had syntax highlighting at one point but if I remember correctly we had to remove it in the course of adding some other feature. Restoring syntax highlighting would be perfect!

[kukimik]

if I remember correctly we had to remove it in the course of adding some other feature

It was removed here: #1959.