feat(docs): Add battery sensor documentation

docs/docs/features/battery.md

@@ -0,0 +1,38 @@
+---
+title: Battery Level
+sidebar_label: Battery Level
+---
+
+If your keyboard has a battery sensor, ZMK will report its battery level to the connected bluetooth host and show it on the keyboard's display, if it has one.
+
+For split keyboards, only the battery level of the central (usually left) side is reported over bluetooth.
+
+:::note
+
+Windows may not properly ask the keyboard to notify it of changes in battery level, so the level shown may be out of date.
+
+:::
+
+## Adding a Battery Sensor to a Board
+
+To enable a battery sensor on a new board, add the driver for the sensor to your board's `.dts` file. ZMK provides two drivers for estimating the battery level using its voltage:
+
+- `zmk,battery-voltage-divider`: Reads the voltage on an analog input pin.
+- `zmk,battery-nrf-vddh`: Reads the power supply voltage on a Nordic nRF52's VDDH pin.
+
+Zephyr also provides some drivers for fuel gauge ICs such as the TI bq274xx series and Maxim MAX17xxx series. If you use a battery sensor that does not have an existing driver, you will need to write a new driver that supports the `SENSOR_CHAN_GAUGE_STATE_OF_CHARGE` sensor channel and contribute it to Zephyr or ZMK.
+
+Once you have the sensor driver defined, add a `zmk,battery` property to the `chosen` node and set it to reference the sensor node. For example:
+
+```
+/ {
+    chosen {
+      zmk,battery = &vbatt;
+    };
+
+    vbatt: vbatt {
+        compatible = "zmk,battery-nrf-vddh";
+        label = "VBATT";
+    };
+}
+```

docs/sidebars.js

@@ -17,6 +17,7 @@ module.exports = {
       "features/encoders",
       "features/underglow",
       "features/backlight",
+      "features/battery",
       "features/beta-testing",
     ],
     Behaviors: [