ePUB output doesn't have table of contents

[agjohnson]

It seems our docs do not generate an epub file with a table of contents. I opened our docs.readthedocs.io epub and noticed the table of contents was missing in both calibre and foliate:

The underlying issue is the build/epub/toc.ncx is mostly empty.

This definitely makes the format very hard to navigate. I'm sure other projects are hitting this as well, so probably worth investigating. This could be anything from just our docs, to our sphinx theme, any of our extensions, or even sphinx.

• Perhaps an issue with the Sphinx theme?
  • I built an epub on the sphinx theme docs and it did have a table of contents
  • I built an epub on our docs with 'basic' theme and epub_theme="epub", and it still had an empty toc.ncx
  • Our theme is likely not affecting things then
• One of our extensions?
  • We have some extensions that manipulate the docs a good deal, it's not unlikely that we wrote an extension that is interfering with epub
  • I disabled with extensions = [] and didn't notice a change, but this probably needs more testing.
• Something about our doc project?
• Sphinx?