Adds custom targets which take care of generating and installing
documentation files.
Every module has a global set of targets suffixed with the module
name in order for them to be unique when we implement super builds.
The targets are the same as the list below, but replace ${target}
with the module's name. Eg.: docs_qtbase.
For every target which has an qt_add_docs() call, we now create the
following set of custom targets:
* docs_${target}
* html_docs_${target}
* qch_docs_${target}
* prepare_docs_${target}
* generate_docs_${target}
* install_docs_${target}
* install_html_docs_${target}
* install_qch_docs_${target}
Fixes: QTBUG-75859
Change-Id: Ie84cb9a2dedbe7333d9a84f4d73383442deca477
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
