CMake: Introduce QT_BUILD_ONLINE_DOCS flag for documentation builds

QDoc modifies its linking behavior depending on whether the -installdir
command line option was set or not; Without -installdir, QDoc will
use 'https://<url>' to link to targets loaded from external
documentation modules, instead of using relative paths.

Drop the -installdir if QT_BUILD_ONLINE_DOCS was set. This behavior matches
the 'build_online_docs' scope that was used for qmake documentation
targets, and is useful for documentation projects that are not part of
Qt but want to link to Qt docs simply by loading the index files of Qt's
modules.

Pick-to: 6.2
Change-Id: Ib1bbf7b9d784e0c276d2bc520240af4a6c8ba903
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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