Move qdoc part of qtbase

To enable doc builds per submodule the Coin/CI has a chicken-egg problem with qdoc. If we wanted to build qtbase and build also related documentation for it we'd need to build qttools to get the qdoc, which in turn depends on qtbase..

Currently this is "solved" in Coin/CI so that documentation is not built per submodule integration (like it should) but instead we add extra monolithic qt5 build that also builds the documentation. This adds unnecessary builds and resource usage ($$$).

Suggestion: move qdoc to qtbase.git

• This should simplify how CI needs to deal with documentation builds
• Documentation builds can be enabled per submodule integration
  • doc artifacts can be obtained without full qt5 integration
• Marketplace integration
  • Marketplace components are src only, clients building the component documentation need qdoc which is not part of the essentials binary package (essentials binaries are installed via Qt installer): https://doc.qt.io/qt-5/qtmodules.html