Add comprehensive usage examples for StoreCollectionData.AcquiredDate property

[Copilot]

This PR addresses the lack of usage documentation for the StoreCollectionData.AcquiredDate property by adding comprehensive examples and clarifications.

Changes Made

Enhanced storecollectiondata_acquireddate.md:

• Added a complete C# example showing how to access the AcquiredDate property
• Clarified that AcquiredDate represents when the user first acquired the product from the Microsoft Store (not the installation date)
• Explained the difference between acquired date vs installation date
• Added proper error handling in the example
• Included cross-references to related methods like GetUserCollectionAsync

Updated storecollectiondata.md:

• Added a comprehensive example showing how to access StoreCollectionData objects
• Demonstrated the complete flow: StoreContext → GetUserCollectionAsync → StoreSku → CollectionData
• Showed usage of multiple properties (AcquiredDate, IsTrial)

Improved storesku_collectiondata.md:

• Clarified that the property returns null when user has no entitlement
• Added guidance on how to access StoreSku objects with collection data
• Enhanced cross-references to related documentation

Example Usage

The documentation now shows developers exactly how to get the AcquiredDate:

StoreContext storeContext = StoreContext.GetDefault();
string[] productKinds = { "Application", "Durable" };
StoreProductQueryResult queryResult = await storeContext.GetUserCollectionAsync(productKinds);

foreach (StoreProduct product in queryResult.Products)
{
    foreach (StoreSku sku in product.Skus)
    {
        if (sku.CollectionData != null)
        {
            DateTime acquiredDate = sku.CollectionData.AcquiredDate;
            // Use the acquired date...
        }
    }
}

This resolves the user's questions about:

1. How to use the AcquiredDate property with concrete examples
2. How to obtain StoreCollectionData objects (via StoreContext methods)
3. What AcquiredDate represents vs installation date

Fixes #1850.

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