Support headings (H2, H3) inside a <details> tag

[kkbrooks]

This would be most useful in things like VS Code getting started tutorial to have a collapsible section for the main topics, and then have an H2 to have the links in the right rail and H3/H4s for further structuring the contents of the <details>. That way the tutorial page becomes a set of collapsible sections that is very scannable. You then open the one you're interested in.