Add docs publishing job

[mtreinish]

Summary

This commit adds a new CI job for publishing documentation on release. Now that qiskit-aer is a self contained package with it's own namespace the documentation build no longer needs to be combined into the main qiskit docs anymore. As such this commit will enable aer ci to publish built documentation for the project to:

https://qiskit.org/documentation/aer

on every release.

Details and comments

Fixes Qiskit/qiskit#1508

[jakelishman]

If we're publishing it separately this time round, we probably need to run through the whole documentation adding intersphinx to Terra, and making sure the monkey-patched QuantumCircuit methods are still getting documented.

[mtreinish]

Well I was more concerned about how the main docs build will deal with the new namespace. I guess it would work fine and just build the pages as stubs/qiskit_aer.* instead of stubs/qiskit.providers.aer.* and it should work fine. Maybe lets hold off on this until after we remove aer from the metapackage and update https://github.com/Qiskit/qiskit/pull/1596 to redirect in the same docs build. What do you think?

[jakelishman]

I'm mostly concerned with the things like QuantumCircuit.save_statevector that aren't going to be the target of any autosummary directive any more after the move (previously Terra's qiskit.circuit module would have pulled them in as they'd be attached to QuantumCircuit during the build process), and I'm worried about any places in the Aer documentation that might link to Terra objects (e.g. :class:`.QuantumCircuit` will fail in a separate build).

I think the stubs that are generated by the Aer build itself should work, as you say.

[jakelishman]

Happy to hold off on making changes to the documentation build process til later - we don't necessarily need to hold Aer 0.11 on that.