Feature: RGBLight layers (#7768)

Feature: RGBLight layers (#7768)

* New feature: RGBLIGHT_LAYERS

This feature allows users to define multiple independent layers of lighting
that can be toggled on and off individually, making it easy to use your
RGB lighting to indicate things like active keyboard layer & modifier state.

* Demonstrate built in functions for layer state checking

Also link the video in the docs.

* Follow existing pattern for setting rgblight_status flags

* Eliminate rgblight_is_static_mode since it's not needed

Just check to see if the timer is enabled directly.