Export to flat HTML file?

### What is the problem or limitation you are having?

I have zero idea how hard this would be to implement, but I ran into a situation where the venue's split screen support wasn't working, so we needed to put the slides on a tablet, while I rendered the speaker notes on my laptop.

I handled that by manually creating a LibreOffice slide deck, but wondered how feasible it would be to support exporting pandoc-friendly HTML that could then be further converted to other formats like PDF or OpenDocument's presentation format. (such conversions could also provide a way to prepare slides for printing)

### Describe the solution you'd like

Export to a conversion-friendly HTML format with inline CSS styling.

### Describe alternatives you've considered

Generating PDF or OpenDocument's presentation format directly

### Additional context

_No response_

No files or tests are named. Start by tracing the existing Markdown-based presentation rendering path and how speaker notes are handled, then assess an inline-CSS, pandoc-friendly HTML export against the PDF and OpenDocument alternatives. Done means the export preserves the slide content and notes in a conversion-friendly flat HTML file.

Written by the indexing model from the issue text.