Make documentation available for offline use

Greetings,

this is a ticket per my recent exchange of thoughts with crmaurei on IRC / Gitter.

I suggest that PySide2's documentation be made available for offline use - as a Sphinx project, a Qt Assistant resource, or perhaps both, combined potentially with bundling it in a PyPI wheel.

This would be akin to what, say, Python itself does, where the Sphinx documentation can be easily built locally.

Regards.