Use find_package(Qt6 REQUIRED COMPONENTS ...) idiom

We should change the documentation and examples to use

find_package(Qt6 REQUIRED COMPONENTS Core)

idiom. So far our documentation uses

find_package(Qt6 COMPONENTS Core REQUIRED)

while examples just use

find_package(Qt6 COMPONENTS Core)

1. Reasoning

• REQUIRED enforces a fatal error if Qt6 is not found, which makes the error more obvious to the end user.
• Using REQUIRED before the components list better matches behavior of OPTIONAL_COMPONENTS argument .