ReactiveUI: The Book

This massive PR is the :shipit: PR for the new version of the ReactiveUI documentation. This documentation is intended to be a comprehensive usage guide / reference for the library.

## TODO:

#### Introduction

- [ ] Getting started
- [ ] Library Overview

#### RxUI Core
- [ ] AutoPersistHelper / AutoPersistCollection
- [ ] ActOnEveryObject
- [ ] BindingTypeConverters
- [ ] ICreatesCommandBinding + Implementations
- [ ] Design Mode / Unit Test detection + Unit Test Override
- [ ] UserError
- [ ] Dependency Resolution + ModernDependencyResolver
- [ ] ReactiveUI Startup
- [ ] ICreatesObservableForProperty + Implementations
- [ ] IHandleObservableErrors pattern + DefaultExceptionHandler
- [ ] MemoizingMRUCache
- [ ] Logging
- [ ] MessageBus and how to avoid using it
- [ ] ObservableAsPropertyHelper / Output Properties
- [ ] OrderedComparer
- [ ] Semantics of WhenAny + WhenAnyValue
- [ ] Basic Property Binding 
- [ ] Property Binding => Type Conversion + Binding Hooks + Hints
- [ ] CreateCollection + Testing
- [ ] CreateDerivedCollection
- [ ] ReactiveCommand Basics + WhenAnyObservable
- [ ] ReactiveCommand Async
- [ ] ReactiveList Basics + Semantics of Reset + AddRange optimizations + 
  Change Tracking + Suppress Change Notification
- [ ] ReactiveObject + Suppress Change Notification
- [ ] Reflection Utilities
- [ ] List of Registrable Interfaces + Affinity Pattern
- [ ] View Location + IViewFor
- [ ] Routing
- [ ] Suspension Host
- [ ] WhenNavigatedTo
- [ ] Default Schedulers 

#### RxUI.Events
- [ ] Events
- [ ] Cocoa Delegates

#### RxUI.Mobile
- [ ] Suspension Basics
- [ ] Suspension Host
- [ ] Geolocation

#### RxUI.Platforms Android
- [ ] Android Activity + Fragment
- [ ] Android Scheduler / Initialization Trickery
- [ ] State of Platform

#### RxUI.Platforms Cocoa
- [ ] Observing via KVO
- [ ] Reactive NSView + NSViewController + IViewFor copy-pasting
- [ ] ReactiveTableViewSource
- [ ] iOS Nonsense Tricks
- [ ] State of Platform

#### RxUI.Platforms XAML
- [ ] Auto DataTemplate binding hooks
- [ ] BooleanToVisibilityTypeConverter
- [ ] TransitioningContentControl
- [ ] State of Platform

#### RxUI.Testing
- [ ] Talk about it

#### Misc
- [ ] Map everything to ReactiveCocoa
- [ ] Serialization of ReactiveObject / other things