Fix API document

[kwkbtr]

Fix some problems in API document.

• Document URLs (private preview)

  • C++ API Reference http://docs.qulacs.org/en/fix_doc/api/cpp_library_root.html
  • Python API Reference http://docs.qulacs.org/en/fix_doc/pyRef/modules.html

• Update Sphinx-related libraries

• Fix a problem that some classes (e.g. QuantumCircuit) did not appear on the reference

• Use header files from the submodule qulacs/ instead of the copied version qulacs-src/

  • Note that updates made in #13 are not reflected, so this version is not ready to be made public yet

• Include vqcsim as well as cppsim

[akpc]

This looks good to me!

Just as a note, this repo has a qulacs-src/cppsim dir because we translated the cpp comments at the time, but don't want to create difficult to resolve conflicts with the dev branch of the upstream qulacs codes It would be nice if we can put the translations into the source code and delete qulacs-src/cppsim in the future