.NET MAUI 9: ResourceDictionaries without code-behind have their XAML compiled

[davidbritch]

In .NET 9, all ResourceDictionary instances without code-behind have their XAML compiled, unless <?xaml-comp compile="false" ?> is set.

Update the ResourceDictionary doc to this effect.

https://github.com/dotnet/maui/pull/19390

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Associated WorkItem - 314388