Simplify Hello World apps and update docs (#28)

* Simplify Hello World apps
* Updated docs

app-sim/src/hello_world/hello_world.c

@@ -4,9 +4,6 @@
 
 static void app_show(TT_UNUSED App app, lv_obj_t* parent) {
     lv_obj_t* label = lv_label_create(parent);
-    lv_label_set_recolor(label, true);
-    lv_obj_set_width(label, 200);
-    lv_obj_set_style_text_align(label, LV_TEXT_ALIGN_CENTER, 0);
     lv_label_set_text(label, "Hello, world!");
     lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);
 }
@@ -18,5 +15,6 @@ const AppManifest hello_world_app = {
     .type = AppTypeUser,
     .on_start = NULL,
     .on_stop = NULL,
-    .on_show = &app_show
+    .on_show = &app_show,
+    .on_hide = NULL
 };