MA0109: Skip diagnostic for program entry point Main(string[] args)

[Copilot]

MA0109 was incorrectly flagging Main(string[] args) as needing a Span<T>/Memory<T> overload, despite the entry point signature being fixed by the runtime — there's no valid overload to add.

Changes

• Analyzer (AddOverloadWithSpanOrMemoryAnalyzer): Skip analysis when the method is the compilation's entry point via Compilation.GetEntryPoint(...).
• Tests: Added EntryPoint_Main_ShouldNotTrigger (no diagnostic on Main) and EntryPoint_NonMainMethod_ShouldTrigger (other string[] methods in a console app still report).
• Docs: Updated MA0109.md to document the entry point exception.

// Before: triggered MA0109
public static void Main(string[] args) { }

// After: no diagnostic — entry point signature is runtime-mandated
public static void Main(string[] args) { }