Fix C++ library compile issues and guides

[kotamanegi]

This PR fix the following two issues:

1. C++ library compile issues
   In Eigen 3.3.5 with CUDA, a lot of Eigen users report that compile errors happen in Eigen/Half.h file.
   This qulacs C++ library is being affected by this issue.
   So, I tried updating version to Eigen 3.3.5 -> 3.3.7, which Eigen maintainers say that the issue has been removed.
   I manually checked that this solution passes tests.

2. Usage guide of C++ with CUDA
   When we use nvcc, we need to add many compile options compared to non-GPU qulacs.
   Therefore, I think it is essential to add usage guide on documents.

[corryvrequan]

Sorry for bothering you with out-of-date documents, and thanks for updating our documents.
The updated documents seem correct and nice!