Use `highlight.js` for syntax highlighted code blocks?

[clason]

see neovim/neovim@d1beb90

We don't need the whole package but can cherry pick just the three or four languages we need (Lua, Vim, C, Python; unfortunately, Vim is not in the bundle that is distributed via CDN, so we need to install it locally).

We also probably need to write/adapt the style to match the current site theme as none of the provided ones fit really well.