Fix jump to ToC for nested headers

[zerebos]

This is meant to fix #4228. I saw your comment there about trying to search in arbitrary depth of the ckeditor model #4228 (comment), but I thought why not try to make it use the same method as read-only notes?

I tested this out locally of course and it seems to work as expected for a bunch of different nested cases. But you can just consider this a proposal and feel free to close it if you don't think it's the best way forward.

[zadam]

I kind of expected some trouble because of all the advanced stuff CKEditor does on the background, but if this simple straightforward approach works, all the better ...