Add explanation for contravariance behavior with anonymous functions

[Copilot]

This PR addresses a counterintuitive behavior with lambda expressions and delegate contravariance that was causing confusion for C# developers.

The Problem

The existing documentation explained contravariance well for named methods, but didn't cover why this seemingly equivalent code behaves differently:

// This works
var personReadContact = (Person p) => p.ReadContact();
Action<Employee> employeeReadContact = personReadContact;

// This fails with CS1661 compiler error
Action<Employee> employeeReadContact = (Person p) => p.ReadContact();

The Solution

Added a new section "Contravariance and Anonymous Functions" that explains:

1. The Root Cause: This behavior is due to lambda expression type inference rules, not contravariance itself
2. Why It Happens: The compiler cannot directly infer that (Person p) => p.ReadContact() should have type Action<Person> when being assigned to Action<Employee>
3. Practical Workarounds: Explicit casting and matching parameter types
4. Key Distinction: Difference between delegate contravariance (works after types are established) and lambda type inference (occurs during compilation)

The new documentation includes complete code examples that demonstrate both the problem and the solutions, helping developers understand this subtle but important compiler behavior.

Fixes #31678.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md	Using Variance for Func and Action Generic Delegates (C#)

[BillWagner]

One minor change, and this is ready.

[BillWagner]

This is now ready for final review.