Pe cmake doc build

Do you agree with these changes?

- Documentation will be built now in the build directory using cmake and its macro UseLATEX.cmake.
- Images of documentation are in separate directories.
- Target names unified as suggested.

question: 
- if input_reference.tex is not stable then we can create separate pdf and only if it is built successfully we add it at the and of the documentation as appendix
- when to push it? it will influence building Flow123d on dev and for everyone of us...