Fix smart cards authentication example - show how to obtain adminKey

[Copilot]

The smart cards authentication documentation was incomplete, showing code examples that used an adminKey parameter without explaining how to obtain it. This made the authentication examples essentially unusable for developers.

Problem:
The existing code examples assumed developers already had access to rootPage.AdminKey without any guidance on:

• Where this admin key comes from
• How to generate or obtain it for different scenarios
• How to store and retrieve it for authentication operations

Solution:
Added comprehensive guidance on admin key management:

1. New "Obtaining the admin key" section explaining different scenarios:

   • Virtual smart cards you created (use the same key from creation)
   • Existing physical/virtual cards (provided by IT/card management system)
   • Development/testing (generate with CryptographicBuffer.GenerateRandom(24))

2. Example admin key management methods showing practical implementations:

   • SaveAdminKeySecurely() - stores admin key in app settings with Base64 encoding
   • GetStoredAdminKey() - retrieves stored admin key with proper error handling

3. Updated code examples to use the new management methods:

   • Authentication example now calls GetStoredAdminKey() instead of undefined rootPage.AdminKey
   • PIN reset example includes clear comments about admin key source
   • Connected virtual card creation workflow with subsequent authentication operations

The documentation now provides a complete, working example that developers can actually implement.

Fixes #2253.

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