[13.3] Document command return values

[JamesNK]

Commands can now return a value. e.g.

       .WithCommand(
           name: "migrate-database",
           displayName: "Migrate Database",
           executeCommand: (c) =>
           {
               var markdown = """
                   # ⚙️ Database Migration Summary

                   | Table      | Result                     |
                   |------------|----------------------------|
                   | Customers  | ✅ 1,200 rows              |
                   | Products   | ✅ 850 rows                |
                   | Orders     | ✅ 3,400 rows              |
                   | OrderItems | ✅ 8,750 rows              |
                   | Categories | ✅ 45 rows                 |
                   | Reviews    | ❌ FK constraint violation |
                   | Inventory  | ✅ 850 rows                |
                   | Shipping   | ✅ 3,400 rows              |
                   | Payments   | ❌ Timeout after 30s       |
                   | Coupons    | ✅ 120 rows                |

                   **Summary:** 8 of 10 tables migrated successfully. 2 tables failed.
                   """;
               return Task.FromResult(CommandResults.Success("Database migrated.", new CommandResultData { Value = markdown, Format = CommandResultFormat.Markdown }));
           },
           commandOptions: new() { IconName = "CloudDatabase", Description = "Migrate the database with sample store data" });

Needs to be documented.