Top-level and repo-level doc installation targets are dysfunctional

This is something I noticed when I started the Qt6 packaging:

`make install_docs_<module> DESTDIR=/path/to/buildroot` fails

The best explanation I found is that `add_custom_target(install_html_docs_${target} ...)` in qtbase/cmake/QTDocsHelpers.cmake tries to copy directories rather than installing build targets.

For the moment, I use a patch which creates a real installation target but I'm not happy with the result yet.