Add definitions for expressions and statements to C# fundamentals program structure

[Copilot]

This PR addresses the gap in fundamental C# documentation by adding clear, working definitions for "expressions" and "statements" to the program structure guide. These concepts are essential for understanding C# programming, especially as the language evolves to include more expression-based features.

Changes Made

Added a new "Expressions and statements" section to /docs/csharp/fundamentals/program-structure/index.md that includes:

• Clear definitions: Expressions evaluate to values; statements perform actions
• Concrete examples: 5 examples each to illustrate the concepts
• Key distinction explanation: How to tell expressions from statements
• Edge case coverage: Method calls can be both expressions and statements
• Cross-references: Links to existing detailed documentation

Example Content

Expression examples:

• 42 (literal value)
• x + y (arithmetic operation)
• Math.Max(a, b) (method call)
• condition ? trueValue : falseValue (conditional expression)

Statement examples:

• int x = 42; (declaration statement)
• Console.WriteLine("Hello"); (expression statement)
• if (condition) { /* code */ } (selection statement)

The definitions are placed logically after the program structure explanation but before the Related Sections, making them easily discoverable for beginners learning C# fundamentals.

Fixes #42561.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/program-structure/index.md	docs/csharp/fundamentals/program-structure/index

[BillWagner]

This is a great start. I have a couple suggestions to improve the flow of the text.