Fix build warnings

[Hiroya-W]

/workspaces/qulacs/doc/en/source/apply/5.2_qcl.ipynb:64: WARNING: image file not readable: apply/attachment:img/QCL.png
/workspaces/qulacs/doc/en/source/write/0_readme.rst:76: WARNING: Literal block expected; none found.
checking consistency... /workspaces/qulacs/doc/en/source/write/0_readme.rst:98: WARNING: Citation [note] is not referenced.
/workspaces/qulacs/doc/en/source/intro/4.2_cpp_tutorial.rst:771: WARNING: Pygments lexer name 'txt' is not known

[Hiroya-W]

/workspaces/qulacs/doc/en/source/write/0_readme.rst:76: WARNING: Literal block expected; none found

に関しては、CONTRIBUTINGのドキュメントを更新する必要があるのが本質なので、別のIssueで対応します

[Hiroya-W]

/workspaces/qulacs/doc/ja/source/api/cpp_library_root.rst:1: WARNING: Title overline too short.

=============
C++ APIリファレンス
=============

/workspaces/qulacs/doc/ja/source/guide/2.0_python_advanced.rst:305: WARNING: Title underline too short.

DensityMatrixクラス
----------
/workspaces/qulacs/doc/ja/source/guide/2.0_python_advanced.rst:305: WARNING: Title underline too short.

DensityMatrixクラス
----------
/workspaces/qulacs/doc/ja/source/guide/2.0_python_advanced.rst:1928: WARNING: Title underline too short.

変分回路の勾配を求める
~~~~~~~~~~~~~~~~~~~~
/workspaces/qulacs/doc/ja/source/guide/2.0_python_advanced.rst:1928: WARNING: Title underline too short.

変分回路の勾配を求める
~~~~~~~~~~~~~~~~~~~~
/workspaces/qulacs/doc/ja/source/guide/2.0_python_advanced.rst:1967: WARNING: Title underline too short.

QuantumCircuitSimulator
~~~~~~~~~~~~~~~~~~~~
/workspaces/qulacs/doc/ja/source/intro/1_install.rst:25: WARNING: Title underline too short.

Install Python library from source
-------------------------------
/workspaces/qulacs/doc/ja/source/intro/1_install.rst:25: WARNING: Title underline too short.

Install Python library from source
-------------------------------