Add documentation for object initializer syntax without 'new' keyword

[Copilot]

This PR addresses the missing documentation for the C# object initializer syntax that lacks the new keyword, as specified in C# ECMA spec 12.7.11.3. This feature allows initializing nested objects using = { } syntax without creating new instances.

Changes Made

Enhanced Documentation

• object-and-collection-initializers.md: Added comprehensive section explaining the difference between Property = { ... } and Property = new() { ... } syntaxes
• how-to-initialize-objects-by-using-an-object-initializer.md: Added brief introduction to the syntax without new keyword

New Code Examples

• ObjectInitializerWithoutNew.cs: Created focused examples demonstrating:
  • Difference between modifying existing instances vs. creating new ones
  • Practical use cases with read-only properties
  • Working code that compiles and runs successfully

Key Feature Documented

The ability to initialize objects deeply within properties using = { } syntax without the new keyword:

// Modifies existing instance (no 'new' keyword)
var person = new Person
{
    Name = "Alice",
    Address = { Street = "123 Main St", City = "Anytown" }
};

// Works with read-only properties
var app = new Application
{
    Name = "MyApp",
    AppSettings = { Theme = "Dark", FontSize = 14 }  // AppSettings is read-only
};

This syntax is particularly valuable when:

• Working with read-only properties that don't have setters
• Wanting to modify existing instances rather than replace them
• Initializing nested properties deeply within object hierarchies

All code examples have been tested and compile successfully with .NET 8.

Fixes #12979.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/classes-and-structs/how-to-initialize-objects-by-using-an-object-initializer.md	docs/csharp/programming-guide/classes-and-structs/how-to-initialize-objects-by-using-an-object-initializer
docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers.md	docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers

[BillWagner]

One major change, and this is ready to go.

[BillWagner]

Thanks @copilot

This is now ready for final review.

[BillWagner]

ping @IEvangelist for review