Rewrite getting started, improve tutorials and install pages #1808
This PR contains a rewrite of the getting started page, and improves the tutorials and install docs pages. The index.rst landing page now very prominently points people to first getting started (which is a step by step how to get set up and use gammapy page) and then to the Jupyter tutorials, which is just a listinig of the tutorials we have.
This is already describing the solution that @Bultako has been working on and is finalising at the moment to have
@Bultako - I would prefer to merge this in and then adjust as needed tomorrow and already ask a few people to try this out tomorrow or Friday latest.
But up to you, if you want to review and leave comments here, I can continue to work on this later tonight or tomorrow morning.