Port qt5.git documentation building instructions to CI yaml instructions

Our CI currently has a Documentation CI config defined here
https://code.qt.io/cgit/qt/qt5.git/tree/coin/platform_configs/default.yaml?id=0f7c2c37395cad5a6692123d270be9df2e5be189#n16

This uses a list of instructions that's hardcoded in Coin, which assume a qmake build of Qt.
The instructions boil down to:

/home/qt/work/qt/qt5/configure /path/to/qt5-top-level-src
make module-qttools
make docs
mkdir -p /home/qt/work/build/qtbase/doc/global
cp -rf /home/qt/work/qt/qt5/qtbase/doc/global/ /home/qt/work/build/qtbase/doc # don't know what's the purpose of this

Note the above instructions are only called for the top-level qt5.git work item. For the per-repo work items, the CI simply builds and tests the repos as usual.

Once the qmake configs are removed from CI, these instructions won't work anymore.

It's unclear to me whether this feature is only needed for build coverage (to ensure our build system rules work), or if the actual result of calling 'make docs' is used for our official documentation.

Either way, these instructions most likely need to be ported to Yaml instructions.

Not sure how https://codereview.qt-project.org/c/qt/qttools/+/304695 ties into this