[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.

[IEvangelist]

Triage → @Copilot

Scope: Document command return values, new in Aspire 13.3. WithCommand(...)'s executeCommand callback can now return a value (string / markdown) that the dashboard renders inline.

Where it belongs

• The existing app-host/commands page (or whichever page under src/frontend/src/content/docs/app-host/ covers WithCommand) — add a section "Returning a result from a command" with the markdown-return example from this issue body.
• The dashboard page that covers custom commands — add a small note + screenshot reference about rendered results.

Acceptance

• Confirms the return type / API contract by reading the corresponding microsoft/aspire PR / source (src/Aspire.Hosting/ApplicationModel/ResourceCommandAnnotation*.cs and the dashboard command handler).
• Example renders as the dashboard would (markdown table, etc.).
• 13.3 changelog entry references the new behavior.
• No unrelated edits.