Added missing GPIO0 reference to the soldering section.

Setting the PWM channels and frequencies is now done fully from
the device yaml configuration file, so I removed the frequency
code from the CPP code.

Flashing doc

The work-around has been implemented in a forward-compatible manner,
so the light transition when turning off the light should still
work once the upstream code gets fixed.

* When turning off the light at a low brightness setting,
  the LEDs were seen flashing brightly for a short time.

* Transitioning did not work when turning of the light. The light would be
  kept at normal brightness during the transition and then it turned of
  at once. This looks like an issue in the ESPHome transitioning code,
  because that is the code that is responsible for the transitioning.
  When turning on the light, it works correctly. I see both the state
  and the brightness go up. But when turning off the light, I see that
  the state decreases  gradually to zero, but the brightness is kept
  as-is. I wrote a fix for this behavior in my code and will investigate
  if the ESPHome code can be fixed for this.