Support Sphinx2

[humitos]

Sphinx2 is doing a lot of incompatible changes that we should start considering into our roadmap to support it.

This is not an issue to start working now, but to consider that it will affect our platform in many ways and we should start thinking about it and noting down where the problems would be and what are the actions that we should take.

Related: #3581 (comment)

[stsewd]

I have a PR from the extension side readthedocs/readthedocs-sphinx-ext#57

[stsewd]

And we need #3581, well we only need that if we want to make sphinx 2.0 the default

[davidfischer]

I don't think sphinx2 will be our default for quite some time.

[humitos]

FYI. Sphinx builder dirhtml in Sphinx 2.0+ uses the name dirhtml. I think RTD v2 config is using htmldir. I'm not sure if the two are mapping correctly.

(from #4638 (comment))

[stsewd]

We map the builder from the config file to our custom builder, so if anything we should change our builder custom builder (if needed). Also maybe is a good idea to include dirhtml as option in the config file too?

[stsewd]

Just for clarification, we use our custom builder (readthedocsdirhtml). htmldir was used because that's how all classes are called in our code. So, this is nothing blocking for sphinx 2.0 support :)

[stsewd]

We already support sphinx2. Only the rtd theme is missing support for it.

[nschloe]

@stsewd Could you please link the bug for the rtd theme?

[stsewd]

@nschloe readthedocs/sphinx_rtd_theme#838