Modernize github-lookalike markdown rendering

[isoos]

Our github-markdown.css is outdated in many aspect (e.g. dark mode compatibility, how some blocks are rendered - specifically the blockquote mentioned in #8310), and we should update it.

The most comprehensive and up-to-date CSS file(s) seems to be in https://github.com/sindresorhus/github-markdown-css - but we may not need all of the rules from those files. It is generated by fetching the styles from GitHub, parses them and extracts the relevant rules.

We could use them on a case-by-case basis, or also create a similar extraction logic for the styles or specific rules that are relevant to us (e.g. we don't want to use font-family rules where they are not applicable).

[isoos]

/cc @parlough