Backlight - New option : BACKLIGHT_CAPS_LOCK (#4769)

Backlight - New option : BACKLIGHT_CAPS_LOCK (#4769)

* [BACKLIGHT_CAPS_LOCK_INDICATOR] Use backlight toggling as Caps Lock LED

I've implemented this feature because S65-X doesn't have LED indicators
and the existing led_set_kb(usb_led) function try to use backlight as
indicator but that creates an inconsistency with backlight_config state.

- define `BACKLIGHT_CAPS_LOCK_INDICATOR` to enable Caps Lock indicator
  using backlight (for keyboards without dedicated LED)

- Don't turn off LED indicators when suspend because backlight is
  already turned off. Else led_set(0) will turn back on backlight.

- Documentation has been updated

* [BACKLIGHT_CAPS_LOCK_INDICATOR] Turn off all LED indicators except Caps Lock if BACKLIGHT_CAPS_LOCK_INDICATOR

* [BACKLIGHT_CAPS_LOCK] Rename BACKLIGHT_CAPS_LOCK_INDICATOR

* [BACKLIGHT_CAPS_LOCK] Use new IS_LED_ON method