Add MA0203 to flag <returns> tags on void methods

[meziantou]

Summary

• Add analyzer rule MA0203 to report XML documentation <returns> tags on void methods.
• Document the rule in the README and add the rule metadata to the analyzer pack configurations.
• Cover the new behavior with unit tests for normal, empty, and non-applicable cases.

Testing

• Not run (not requested)
• Added analyzer tests for void methods with <returns> and verified non-void methods and constructors are not flagged