Adds a table of contents in the markdown preview, with a button in the
header to toggle whether to show the TOC. When a user clicks one of the
TOC elements, the preview will scroll to the corresponding heading.
I've also cleaned up some MD preview styling that was inconsistent and
causing the preview to overflow unnecessarily. This also fixes some
terminology in the preview code.
<img width="574" alt="image"
src="https://github.com/user-attachments/assets/abb18ba9-21d3-4315-bdc3-e4bdcca39a4c">
