Fails to include headers in blocks that are nested within other blocks

[kenmanheimer]

The table of contents fails to include headers of blocks that are contained within other blocks.

That includes headers in blocks that are contained within @eeacms/volto-columns-block, in the contents part of a @eeacms/volto-accordion-block, or in an HTML block. These all can include headers that are crucial parts of the document structure.

[kenmanheimer]

This is closely related to plone/volto#3762, "Listing block headline should integrate with TOC", though that one refers to the builtin Volto TOC.