Deploy site using implementation-agnostic URLs

[jugglinmike]

From the Sphinx documentation:

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/.

https://www.sphinx-doc.org/en/master/usage/builders/#sphinx.builders.dirhtml.DirectoryHTMLBuilder

Fixes #124