docs: fix links, add references, rename files (#5138)

docs/overview/timer.rst

@@ -1,3 +1,5 @@
+.. _timer:
+
 ======
 Timers
 ======
@@ -35,7 +37,7 @@ For example:
      /*Do something with LVGL*/
      if(something_happened) {
        something_happened = false;
-       lv_btn_create(lv_screen_active(), NULL);
+       lv_button_create(lv_screen_active(), NULL);
      }
    }
 
@@ -138,5 +140,7 @@ If you just want to delete an object and don't need to clean anything up
 in ``my_screen_cleanup`` you could just use :cpp:func:`lv_obj_delete_async` which
 will delete the object on the next call to :cpp:func:`lv_timer_handler`.
 
+.. _timer_api:
+
 API
 ***