- Make all specific problems H2-level headings.
- Create a `doc-toc` div at the top for the table of contents.
  - Don't use `learn-doc` because we may want to style/show this one independently.
- Link to all problem headings.

Note: can't use metadata in a page header because our CI tools splat their own headers into these pages, so anything we add becomes part of the page body.