Create user documentation for ReactiveUI

This PR is meant to create documentation for users of ReactiveUI, to explain how to use the different components, best practices and more, to get up and running quickly.

Most of the contant is based on the current docs on reactiveui.net, but I updated some parts. Not everything has been moved yet.

Things I'd like to add are:

- [ ] How to use WhenAny to create derived properties.
- [ ] How to use the binding system with custom converters, data templates etc.
- [ ] How to deal with the large amount of disposables and when to Dispose certain things like bindings.
- [ ] How to use RxUIViewModelGenerator.
- [ ] How to integrate a custom IoC container.

I can contribute content for some of these, but definitely not all of them. I'm sure others can.