Prepare markup for syntax highlighting?

[hidde]

To do syntax highlighting, we'll need to indicate in each code example what language it is in, and for some, if they are in multiple languages, split them into multiple. We could then apply something like Prism, with an accessible color scheme.

Suggestion:

• use code elements with pre elements inside of them, add (programming) language definition with lang-* pattern described in HTML spec, the code element
• retroactively add this around all code examples in Techniques and Understanding docs

[hidde]

We're now doing that automatically without indication of language.