If an entity occurs directly in markdown, we parse and insert it
directly; but if it occurs in an HTML block, it has to be added to the
HTML accumulator string for deferred parsing when the HTML block ends.
Pick-to: 6.2
Fixes: QTBUG-91222
Fixes: QTBUG-94245
Change-Id: I0cf586d68d6751892ca035a98f77cd67950d3bc4
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>