docs: Add documentation for power module

Add documentation for the power module.

Signed-off-by: Jan Tore Guggedal <jantore.guggedal@nordicsemi.no>

Author: jtguggedal

docs/modules/power.md

@@ -1,11 +1,65 @@
 # Power module
 
-What does this module do
+The power module manages power-related functionality for devices with nPM1300, like the Thingy:91 X, including the following:
+- Monitoring battery voltage and calculating remaining battery percentage.
+- Handling VBUS (USB) connect and disconnect events to enable or disable UART peripherals.
+- Publishing battery percentage updates via zbus messages.
 
 ## Messages
 
+The Power module defines and communicates on the `POWER_CHAN` channel.
+
+### Input Messages
+
+- **POWER_BATTERY_PERCENTAGE_SAMPLE_REQUEST**
+  Requests a battery percentage sample.
+
+### Output Messages
+
+- **POWER_BATTERY_PERCENTAGE_SAMPLE_RESPONSE**
+  Contains the calculated battery percentage.
+
+The power message structure is defined in `power.h`:
+
+```c
+struct power_msg {
+	enum power_msg_type type;
+
+	/** Contains the current charge of the battery in percentage. */
+	double percentage;
+};
+```
+
 ## Configurations
 
-Kconfig and device tree
+The following Kconfig options control this module’s behavior:
+
+- **CONFIG_APP_POWER**
+  Enables the Power module.
+
+- **CONFIG_APP_POWER_DISABLE_UART_ON_VBUS_REMOVED**
+  If enabled, suspends UART devices when VBUS is removed.
+
+- **CONFIG_APP_POWER_THREAD_STACK_SIZE**
+  Size of the Power module’s thread stack.
+
+- **CONFIG_APP_POWER_WATCHDOG_TIMEOUT_SECONDS**
+  Defines the watchdog timeout for the module. Must be larger than the message processing timeout.
+
+- **CONFIG_APP_POWER_MSG_PROCESSING_TIMEOUT_SECONDS**
+  Maximum time spent processing a single message.
+
+See the `Kconfig.power` file in the module's directory for more details on the available Kconfig options.
+
+## Kconfig and device tree
+
+- The Power module uses `npm1300_charger` as specified by device tree.
+- The two UART devices `uart0_dev` and `uart1_dev` defined in device tree will be enabled or disabled based on VBUS events.
 
 ## State diagram
+
+The Power module uses a minimal state machine with a single state, **STATE_RUNNING**. In this state, the module:
+1. Initializes the charger and optional fuel gauge in the state entry function.
+2. Waits for messages to arrive via zbus.
+3. Handles battery sample requests and publishes results.
+4. Monitors for VBUS events to enable or disable UART devices.