Add collapsable markdown sections to html export

[LukeDRussell]

Description

I'd like to be able to collapse markdown blocks in the output HTML, the same as the editor.

I want to be able to use the same artefact for 'higher ups' (non-technical) and technical receivers. I want to have expanded

# Executive Summary
words
words
words

But have collapsed

# Analysis
...

Suggested solution

Bonus points if we can mark levels to be collapsed on open.
E.g. All Level 1 (#) headings are shown, but level 2 (##) and below are hidden.

Are you willing to submit a PR?

• Yes

Alternatives

No response

Additional context

No response

[mscolnick]

This makes sense. We probably won't make it the default but it can be configurable. We don't have the bandwidth right now to take this on, but can track feedback/thumbs-up to help prioritize.