Add tooling for qdoc-based API documentation

This adds the necessary glue for generating API documentation using qdoc

The process roughly works like this:

• a patched qdoc and ECM is built
• the relevant modules are cloned
• CMake is run for each module. This is needed because the actual qdoc invokation is integrated into the build system via ECM. This is reusing the infrastructure for Linux CI builds
• The prepare phase is run for each module and the index files are collected into one folder
• The generate phase is run
• HTML files are published

See frameworks/extra-cmake-modules!457 and https://invent.kde.org/nicolasfella/doc-includes