document how to highlight code in code blocks

[ntno]

• highlight.js - see document code highlighting with highlight.js #148
• pymdown - see PyMdown + Pygments code highlight example #178, PyMdown + Pygments code highlight example (dark theme) #180
• add pymdown to overview
• link to overview from code elements page
• link to overview from palette page

see also:

• https://www.codeinsideout.com/blog/site-setup/markdown-syntax/#code-blocks
• https://squidfunk.github.io/mkdocs-material/reference/code-blocks/