add plotly

Author: eisuke0912

_config.yml

@@ -20,4 +20,5 @@ sphinx:
     # https://jupyterbook.org/en/stable/interactive/interactive.html#plotly
     html_js_files:
       - https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.7/require.min.js
+      - https://cdn.plot.ly/plotly-latest.min.js
     suppress_warnings: ["mystnb.unknown_mime_type"]