Allow linking custom JavaScript files to html builds

[choldgraf]

Some authors may wish to load custom JavaScript with their sites. For example if there is a non-standard visualization library that can only work if you load JS manually on the page.

In Sphinx this is accomplished with the html_js_files flag.

[nthiery]

And in JupyterBook: https://jupyterbook.org/en/stable/advanced/html.html#custom-css-or-javascript