Document WithCommand return-value payloads (Text/Json/Markdown) for Aspire 13.3

[Copilot]

Aspire 13.3 added support for returning payload data from resource commands, including Markdown rendered in the dashboard response viewer. This PR updates the relevant docs to clearly describe the API contract, dashboard behavior, and release-note messaging.

• Custom resource commands docs

  • Renames the core section to “Returning a result from a command”.
  • Adds a focused Markdown return example using CommandResultData with CommandResultFormat.Markdown.
  • Aligns wording around ExecuteCommandResult.Data and result formats.

• Dashboard docs

  • Adds a notification-center note clarifying that Markdown command results are rendered as formatted Markdown in the text visualizer.
  • Reuses the existing response-view screenshot context to anchor the behavior.

• 13.3 changelog

  • Updates the 13.3 AppHost entry to explicitly call out structured command payloads (Text, Json, Markdown) and the CommandResults.Success(string, CommandResultData) usage pattern.

builder.AddProject<Projects.MyService>("myservice")
    .WithCommand(
        name: "migrate-database",
        displayName: "Migrate Database",
        executeCommand: _ =>
        {
            var markdown = """
                # ⚙️ Database Migration Summary
                | Table | Result |
                |---|---|
                | Customers | ✅ 1,200 rows |
                """;

            return Task.FromResult(CommandResults.Success(
                "Database migrated.",
                new CommandResultData
                {
                    Value = markdown,
                    Format = CommandResultFormat.Markdown
                }));
        });