QtHelp: Add a libFuzzer target for registering documentation

Documentation files may come from third parties, and they are parsed
when they are registered in a collection file and whenever their
contents are read. Add a fuzz target which registers the input as a
documentation file in an empty collection file and reads back all of
its files, keywords and contents.

Like the fuzz targets of the other modules, it is a standalone project
built with the manual workflow described in qtbase/tests/libfuzzer.

Task-number: QTBUG-150690
Pick-to: 6.12
Change-Id: I3dbf6d7d02ccd4bab150c5da6deda9577f3b7bc7
Reviewed-by: Kai Köhne <kai.koehne@qt.io>