feat: Add LED indicator support (#3239)

This adds a new zmk,indicator-leds device, which maps HID indicator
states onto any devices that implement the LED driver API. This adds
support for things like a caps lock LED.

The name was chosen so that more drivers could be added later as
zmk,indicator-*, for example a version that uses the LED strip API.

docs/sidebars.js

@@ -38,6 +38,7 @@ module.exports = {
         "features/split-keyboards",
         "features/debouncing",
         "features/battery",
+        "features/led-indicators",
         "features/low-power-states",
         "features/encoders",
         "features/pointing",
@@ -126,6 +127,7 @@ module.exports = {
         "config/combos",
         "config/displays",
         "config/encoders",
+        "config/led-indicators",
         "config/lighting",
         "config/pointing",
         "config/keymap",
@@ -187,6 +189,7 @@ module.exports = {
               items: [
                 "development/hardware-integration/lighting/underglow",
                 "development/hardware-integration/lighting/backlight",
+                "development/hardware-integration/lighting/led-indicators",
               ],
             },
           ],