html_context
https://www.sphinx-doc.org/ja/master/usage/configuration.html#confval-html_context
#設定_(Sphinx)
A dictionary of values to pass into the template engine's context for all pages.
テンプレートエンジンに渡す変数定義
Single values can also be put in this dictionary using
sphinx-build
's --html-define command-line option.