Address Qt 6.8 CMake API review notes for qtprotobuf

https://doc-snapshots.qt.io/qt6-dev/qt-add-protobuf.html

https://doc-snapshots.qt.io/qt6-dev/qtprotobufgen-qt-tool.html

*Action points:*
AP: Update doc string for `EXTRA_NAMESPACE` -> https://codereview.qt-project.org/c/qt/qtgrpc/+/568819
AP: Re-arrange snippets and recheck regarding QML_URI whether it is required
AP: Check if `QML_URI` issues warning if target already exists, rephrase as appropriate -> The explanation is in the text. The snippet was not at the right place, though.
AP: Mention the .proto file names beforehand in the `Resolving dependencies between protobuf targets` doc section
AP: Consider simplifying / rephrasing the PUBLIC link blue not in the same section