updated linux installation documentation

@vpisarev,

I updated Linux installation document mostly for instructions how to use opencv_contrib dataset module for challenge and some other descriptions.
I also think maybe to move all installation guides, placed here:
http://docs.opencv.org/master/doc/tutorials/introduction/table_of_content_introduction/table_of_content_introduction.html
http://docs.opencv.org/master/doc/py_tutorials/py_setup/py_table_of_contents_setup/py_table_of_contents_setup.html
to one topic. Please, let me know what do you think about it, we can create separate PR for that.