Have you clone opencv?
Yes, and opencv_contrib also.
I did as stated in https://docs.opencv.org/master/d4/db1/tutorial_documentation.html, :
Generate documentation
Get the OpenCV sources
Get the OpenCV_contrib sources
Create build directory near the sources folder(s) and go into it
Run cmake:
cmake -DBUILD_DOCS=ON -DOPENCV_EXTRA_MODULES_PATH=../opencv_contrib/modules ../opencv
delete cmakecache.txt start cmakegui
And do what with it?, (I never used it before)