Essentials

[davidbritch]

Migrate the Xamarin.Essentials docs to this repo, with updated namespaces, instructions etc.

Essentials is part of MAUI, rather than a separate product, and the content should live under /docs/platform-integration/essentials.

Note that at the time of writing, the ConfigureEssentials method that should be used in app startup is not in the correct namespace and so isn't easily callable.

[davidbritch]

Also, Essentials has its own ms.technology value in MAUI (dotnet-essentials), so docfx.json will need updating so specify it for the folder path.