Add sub-headings to TOCs

[mnelson4]

Eg h2s and h3s under articles. This should be an option of each design.

Here's how to do that with Prince: https://www.princexml.com/doc/prince-output/#bookmark-levels

If enabled, sub-headings should appear a level below their section's title. E.g., .pmb-depth-0's h2s should have depth 2 (whereas the h1 title had depth 1); and .pmb-depth-1's h2s should have depth 3. So .pmb-depth-0 h2 and .pmb-depth-1 h1 both get depth 2.