Automate pushing rendered documentation

[diego-plan9]

In a similar way that terra documentation was handled, we should set up the CI and the project so documentation ends up being pushed to qiskit.org. Ideally:

• allow keeping both the "stable" documentation (the one relevant to published packages) and the "master" (the one for more bleeding edge users) available (and maybe older versions too?).
• for chapters where the main content is rendered from notebooks, automate their conversion.

[jaygambetta]

@diego-plan9 and @abdonrd I'm fine if we start doing this now. Each time this repo is updated the doc pushes from docs to qiskit.org/documentation

[diego-plan9]

Sounds good!

[diego-plan9]

After #72, the script for deploying and the travis integration is in place, albeit currently is set to push to a fake directory (and thus not reaching the landing site yet). Next steps are:

• final tests and deploying the first version of the new documentation
• add support for handling both master and stable documentation versions

[diego-plan9]

Closing for tracking the remaining issue at #80.