Generate some basic install(TARGETS) commands for executables and
libraries. Their contents is based on the example snippets of the
qt_generate_deploy_app_script documentation.

There is no one-true-way to do installation though. For example for a
macOS app bundle project, the developer will likely need to install
the libraries into the bundle's Frameworks directory, and not into
CMAKE_INSTALL_PREFIX/lib.

Nevertheless, having some sensible default install commands provides
some scaffolding for developers to refine upon.

To ensure variables like CMAKE_INSTALL_BINDIR are available (which
are used by the install commands), include GNUInstallDirs in the
generated prelude.
Given that t is usually added by the implementation of
qt_standard_project_setup, add the rest of its implementation into the
prelude as well.
Advertise that the code block can be replaced with
qt_standard_project_setup when using with Qt 6.3

Also advertise usage of qt_generate_deploy_app_script for executable
projects.