Streamline first-time-users experience

[gonzalocasas]

This involves both documentation and coding work, for instance:

• [ ] A fast Getting started video/gif should be present in the main page of the doc/README
• [ ] Documentation should be extended to ensure there's a linear sequence of steps to get started with each of the backends (independently of each other).
• [ ] Generate/publish a cheatsheet with the most important concepts and API (similar to what we started here but updated and ideally generated with LaTeX in this repo instead of using an external service)
• [ ] Consider if a conda constructor standalone installer package makes sense (https://github.com/conda/constructor)
• [ ] Add GH documents at different levels of complexity, instead of one that does it all. Ideally also create components out of the existing code (partially in progress already).