docs: Improve dark mode syntax highlighting (#18002)

This PR introduces [GitHub
Light](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github.css)
and [GitHub
Dark](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github-dark.css)
as the syntax highlighting themes for the corresponding modes.

Release Notes:

- N/A