Relax syncqt dependencies for documentation generation

Currently when doing a top-level build and running ninja html_docs_Core we run 70+ ${target}_sync_all_public_headers custom targets on each re-run of docs generation.

We should rewrite this, so that html_docs_Core depends only on its own ${target}_sync_all_public_headers and make ${target}_sync_all_public_headers depend on the other relevant ${dependency}_sync_all_public_headers targets where ${dependency} would be each module that ${target} depends on.