[ios][docs] Document how iOS library mode is supported

[ivanpovazan]

Both MonoAOT and NativeAOT now support building iOS libraries that do not depend on iOS-specific API (no dependency on Xamarin). We should document how users can build their libraries for this scenario.

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Issue Details

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Both MonoAOT and NativeAOT now support building iOS libraries that do not depend on iOS-specific API (no dependency on Xamarin). We should document how users can build their libraries for this scenario.

Author:	ivanpovazan
Assignees:	ivanpovazan
Labels:	documentation, os-ios
Milestone:	9.0.0