qdoc creates proxy pages without any warning

When qdoc encounters a relates entry which cannot be resolved to an existing page, it will create a "Proxy Page" for it (see e.g. https://doc-snapshots.qt.io/qt6-6.2/qqmlextensionplugin-proxy.html).

As far as I can tell, those do not trigger any warnings, which could cause the doc build to fail. Given that creation of "Proxy Pages" normally is an indication of a user error (typo, forgotten namespace, ...), qdoc should at least create a warning for them – if not outright reject the creation of such pages.