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:

```
[ ]   ├── getting-started.md
[ ]   ├── library-overview.md
[X]   ├── migrating-from-rxui4.md
[ ]   ├── overview.md
[ ]   ├── rac-info.md

      ├── advanced
[ ]   │   ├── act-on-every-object.md
[ ]   │   ├── autopersist.md
[ ]   │   ├── create-derived-collection.md
[ ]   │   ├── handle-observable-errors.md
[ ]   │   ├── index.md
[ ]   │   ├── memoizing-mru-cache.md
[ ]   │   ├── orderedcomparer.md
[ ]   │   ├── property-binding-continued.md
[ ]   │   ├── reflection-utils.md
[ ]   │   ├── serialization.md
[ ]   │   ├── suspension-host.md
[ ]   │   ├── testing.md
[ ]   │   └── when-navigated-to.md

      ├── android
[ ]   │   ├── activities-and-fragments.md
[ ]   │   ├── initialization.md
[ ]   │   └── platform-info.md

      ├── basics
[X]   │   ├── binding.md
[X]   │   ├── create-derived-collection.md
[X]   │   ├── dependency-resolution.md
[X]   │   ├── errors.md
[ ]   │   ├── index.md
[X]   │   ├── logging.md
[X]   │   ├── messagebus.md
[X]   │   ├── reactive-command-async.md
[X]   │   ├── reactive-command.md
[X]   │   ├── reactive-list.md
[X]   │   ├── reactive-object.md
[ ]   │   ├── routing.md
[ ]   │   ├── rx-cocoa-delegates.md
[ ]   │   ├── rx-events.md
[ ]   │   ├── schedulers.md
[ ]   │   ├── testing.md
[ ]   │   ├── to-property.md
[ ]   │   ├── unit-test-detection.md
[ ]   │   ├── view-location.md
[ ]   │   └── whenany.md

      ├── cocoa
[ ]   │   ├── initialization.md
[ ]   │   ├── kvo-observation.md
[ ]   │   ├── platform-info.md
[ ]   │   ├── reactive-view-controllers.md
[ ]   │   └── reactivetableviewsource.md

      ├── customization
[ ]   │   ├── binding-type-converters.md
[ ]   │   ├── creates-command-binding.md
[ ]   │   ├── creates-observable-for-property.md
[ ]   │   ├── dependency-resolution.md
[ ]   │   ├── index.md
[ ]   │   ├── registrable-interfaces.md
[ ]   │   └── startup.md

      └── xaml
[ ]       ├── auto-data-template.md
[ ]       ├── booleantovisibilitytypeconverter.md
[ ]       ├── platform-info.md
[ ]       └── transitioning-content-control.md
```