This directory keeps files used to build the documentation. Documentation itself is built by Doxygen and it is kept in separate git submodule.
You can initialize this submodule by issuing following commands.
git submodule init
git submodule update
Alternatively you can build documentation by executing Make command from top level project directory, providing that you have all necessary tools.
make doc
You have to build the project beforehand as Qbs module is responsible for generation of documentation artifacts!
You can also view documentation online.