Right side TOC will not autogenerate correctly if Markdown subsection headings dont make sense; find all such pages

[felker]

Chief example being if there are two # Heading level 1 AKA <h1> in the page

See #649. Before fixing it:

After:

Not sure how/if we can auto-detect such pages.

I wonder if this type of hierarchy would also break?

# heading 1
## subheading 2
#### subheading 4 (skipped 3)