add a multi-page HTML exporter

[nodiscc]

After #15; define and add a html_multipage exporter that outputs a multipage sphinx site. The directory structure should probably look something like this

├── out
│   ├── html
│   │   ├── licenses.html
│   │   ├── platforms
│   │   │   ├── php.html
│   │   │   └── python.html
│   │   ├── platforms.html
│   │   ├── singlepage.html
│   │   ├── tags
│   │   │   ├── archiving-and-digital-preservation.html
│   │   │   └── bookmarks-and-link-sharing.html
│   │   ├── tags.html
│   │   └── toc.html
│   └── markdown
│       └── singlepage.md

• https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx
• https://stackoverflow.com/questions/47624762/how-to-get-a-list-of-warnings-from-sphinx-compilation/47657926#47657926

[nodiscc]

Fixed in #66