Address review of QT_DEPLOY_foo variables for 6.3

https://doc-snapshots.qt.io/qt6-dev/cmake-variable-reference.html#qt6-core

QT_DEPLOY_BIN_DIR
QT_DEPLOY_LIB_DIR
QT_DEPLOY_PLUGINS_DIR
QT_DEPLOY_PREFIX
QT_DEPLOY_QML_DIR
QT_DEPLOY_SUPPORT

AP: Add clarifying documentation when one would use them
AP: Add a landing page with overview of all the variables, part of the CMake Manual overview deployment task.
AP: Add notice that to modify them some of the vars, one should set CMAKE_INSTALL_Foo at configure time. An alternative is to set them at install time with install(SCRIPT)
AP: Add example how to modify the values using install(SCRIPT)
AP: Add a link to upstream CMake manual what's the point of DESTDIR (for QT_DEPLOY_PREFIX)