Android Documentation Layout

There is no way we are migrating over all of the Xamarin.Android documentation, which in turn means we can simplify the hierarchy significantly.

So what should things look like? 😁

I think we should take "spiritual guidance" from:

* the C# documentation tree: https://github.com/dotnet/docs/tree/f7b99d3c6b02350781b56a454356053787b7aa26/docs/csharp
* Entity Framework Core: https://learn.microsoft.com/en-us/ef/core/

Thus:

* .NET for Android
* Welcome!
* What's new in .NET for Android 8.0
* Breaking Changes (if applicable) / To Be Aware Of (warnings which will become errors XA1039 for Android Support)
* The plan for .NET for Android 9.0
* Getting Started /
* Installation
* tour / `dotnet new` + build + install + run + Activity subclass
* .NET for Android for Java developers
* Releases /
* 8.0, …
* Features /
* Bindings
* Layout Code Behind
* Slim Bindings
* Building Apps /
* Build Items
* Build Properties
* Build Targets
* Message Reference /
* Advanced Topics /
* GC
* JNI internals
* Troubleshooting /
* Debugging JNI Object References