docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/modules.cs

@@ -0,0 +1,16 @@
+public class MyModule : InteractionModuleBase
+{
+    private readonly MyService _service;
+
+    public MyModule(MyService service)
+    {
+        _service = service;
+    }
+
+    [SlashCommand("things", "Shows things")]
+    public async Task ThingsAsync()
+    {
+        var str = string.Join("\n", _service.Things)
+        await RespondAsync(str);
+    }
+}