feat(docs): migrate from .md to .rst (#4129)

docs/porting/timer_handler.rst

@@ -0,0 +1,52 @@
+.. _timer:
+
+=============
+Timer Handler
+=============
+
+To handle the tasks of LVGL you need to call :cpp:func:`lv_timer_handler`
+periodically in one of the following:
+
+- *while(1)* of *main()* function
+- timer interrupt periodically (lower priority than :cpp:func:`lv_tick_inc`)
+- an OS task periodically
+
+The timing is not critical but it should be about 5 milliseconds to keep
+the system responsive.
+
+Example:
+
+.. code:: c
+
+   while(1) {
+     lv_timer_handler();
+     my_delay_ms(5);
+   }
+
+If you want to use :cpp:func:`lv_timer_handler` in a super-loop, a helper
+function :cpp:func:`lv_timer_handler_run_in_period` is provided to simplify
+the porting:
+
+.. code:: c
+
+   while(1) {
+       ...
+       lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+       ...
+   }
+
+In an OS environment, you can use it together with the **delay** or
+**sleep** provided by OS to release CPU whenever possible:
+
+.. code:: c
+
+   while (1) {
+       lv_timer_handler_run_in_period(5); /* run lv_timer_handler() every 5ms */
+       my_delay_ms(5);                    /* delay 5ms to avoid unnecessary polling */
+   }
+
+To learn more about timers visit the `Timer </overview/timer>`__
+section.
+
+API
+***