Update documentation about frequency setting for ESP32 S2/S3/C3 chips

Check the updated README where is new info for ESP32 S2/S3/C3 chips.

This release should fix a compatibility issue with ESP32-S2/S3/C3, where the resolution was configured to too high in ledcSetup and frequency too low. For these chips, the minimal frequency seems to be 200 Hz which you can set in attach(...) function.

The changes are only tested on an ESP32-S3-DevKitC-1. This should solve issue #18.

Added this functionality:

• Configurable frequency
  • Defaults to 50 hz
  • Also tested at 333hz which is common for modern servos.
• Users can now instantiate ServoFloat and ServoDouble in addition to Servo.
  • This is to allow using radians, which would get truncated with integers. Also when using degrees this is beneficial if you want better resolution than 1 degree.
  • The implementation must be in the header file for template classes.
  • ServoBase class is shared between all template versions of ServoTemplate, so the static variable channel_next_free is shared.
  • Added an example for how to use ServoFloat with radians.

Thank you for your contribution @mikaeltulldahl and @h2zero

Fix Servo::attached():

• fix Servo::attached() return value after Servo::detach() called