class sphinx.builders.dirhtml.DirectoryHTMLBuilder
This is a subclass of the standard HTML builder. Its output is a
directory with HTML files, where each file is called index.html and
placed in a subdirectory named like its page name. For example, the
document markup/rest.rst will not result in an output file
markup/rest.html, but markup/rest/index.html. When generating links
between pages, the index.html is omitted, so that the URL would look
like markup/rest/.
From the Sphinx documentation:
https://www.sphinx-doc.org/en/master/usage/builders/#sphinx.builders.dirhtml.DirectoryHTMLBuilder
Fixes #124