v2.1.0 for new PWM_manual example, etc.

### Releases v2.1.0

1. Add example [PWM_manual](https://github.com/khoih-prog/Portenta_H7_PWM/tree/main/examples/PWM_manual) to demo how to correctly use PWM to generate waveform. Check [About DCValue in setPWM_manual #2](khoih-prog/AVR_PWM#2)
2. Add function `setPWM_DCPercentage_manual()` to facilitate the setting PWM DC manually by using `DCPercentage`
3. Optimize for PWM speed when changing `dutycycle` while using same `frequency`

examples/PWM_manual/PWM_manual.ino

@@ -75,8 +75,8 @@ void loop()
   // Update DC every UPDATE_INTERVAL (1000) milliseconds
   if (millis() > update_timeout)
   {
-    //setPWM_DCPercentage_manual(pwm, myPin, dutycyclePercent);
-    setPWM(pwm, myPin, frequency, dutycyclePercent);
+    setPWM_DCPercentage_manual(pwm, myPin, dutycyclePercent);
+    //setPWM(pwm, myPin, frequency, dutycyclePercent);
 
     dutycyclePercent += DCStepPercent;