Close #78 Documentation FindSplash CMake Module

Adds Documentation how to use `find_package(Splash)`.

Fixes the include directory of `splash/splash.h` in the user manual.

Links will be valid after https://github.com/ComputationalRadiationPhysics/picongpu/pull/304 was merged.

- [x] Shall we add a section about this module in the UserManual, too?
- [x] Shall I recompile the pdf?