[Development] Documentation test builds in CI

[Alexandru Croitor]

Hi, 
> On 21. Oct 2021, at 21:40, Topi Reiniö <Topi.Reinio at qt.io> wrote:
> 
> Hello!
> The testing cycle in Qt's CI now includes a documentation build step for submodules of the qt5 repository, in dev branch. What this means is that integrations may fail if they introduce new documentation warnings as reported by QDoc, and you are greeted with a "Documentation check failed" message in gerrit.

Cool.

>   * qt/qtbase and qt/qttools already have this step enabled. We're enabling it for the rest of the modules as we clear them of existing warnings. This may take a bit of time as we also need to update the qtbase dependency for some of the less-frequently trafficked repos.

How do qtbase module integrations have their docs tested if qdoc is not built at that point?
I'm guessing there's a pre-provisioned qdoc for each host platform?