Revisit how we build and use qdoc for documentation building

As far as i understand, currently we build qdoc in a Jenkins job, which is then manually provisioned in qt5.git. Then each repo's documentation is built using that qdoc.

This means slow iteration times until a new qdoc can be provisioned, because we usually have various blockages in qt5.git.

An alternative approach, would be to clone a separate qttools:dev + its consistent sha1 dependencies in a qtbase build , build it, and use it for docs. Do the same for other repos.
Somehow cache the qdoc binary.

This would reduce the time needed to wait until a new qdoc is provisioned.

This has some open questions about how to co-ordinate the versioning around the difference sha1s.