docs: main: Add documentation for main module

Add docs for main module.

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

Author: jtguggedal

app/src/Kconfig.main

@@ -12,10 +12,6 @@ config APP_MODULE_TRIGGER_TIMEOUT_SECONDS
 	  Timeout for the trigger timer. On timeout, the module will send triggers for
 	  sensor sampling, location search and polling of shadow and FOTA status from cloud.
 
-config APP_MODULE_THREAD_STACK_SIZE
-	int "Thread stack size"
-	default 3200
-
 config APP_REQUEST_NETWORK_QUALITY
 	bool "Request network quality"
 	help

docs/modules/main.md

@@ -1,11 +1,105 @@
-# Main module
+# Main Module
 
-What does this module do
+The Main module serves as the central control unit of the Asset Tracker Template application. It implements a hierarchical state machine that coordinates the activities of all other modules through [zbus](https://docs.nordicsemi.com/bundle/ncs-latest/page/zephyr/services/zbus/index.html) messages.
+This module handles the application's business logic, including cloud connectivity, data sampling, firmware updates, configuration updates, and user interactions.
 
 ## Messages
 
+The main module does not implement messages available to other modules. Instead, it processes messages from other modules to control the application's behavior.
+It subscribes to messages on the following zbus channels:
+
+The Main module uses the following zbus channels, both for subscribing to incoming data and publishing outbound requests or status updates:
+
+* **BUTTON_CHAN**
+  - Processes user button presses for manually triggering data samples.
+
+* **CLOUD_CHAN**
+  - Receive connectivity status (connected, disconnected) and cloud response data.
+  - Trigger device shadow polling to retrieve configuration updates.
+
+* **ENVIRONMENTAL_CHAN**
+  - Request sensor data from the environmental module.
+
+* **FOTA_CHAN**
+  - Poll for FOTA updates and manage the FOTA process.
+  - Apply FOTA updates to install the new firmware image.
+
+* **LED_CHAN**
+  - Update LED pattern to indicate system state.
+
+* **LOCATION_CHAN**
+  - Requests new location data when a sample is due.
+
+* **NETWORK_CHAN**
+  - Control LTE network connection.
+  - Track cellular connectivity events.
+  - Request network quality samples.
+
+* **POWER_CHAN**
+  - Request battery status.
+  - Initiate low-power mode.
+
+* **TIMER_CHAN**
+  - Handle timer events for sampling.
+
+
 ## Configurations
 
-Kconfig and device tree
+The Main module can be configured using the following Kconfig options:
+
+* **CONFIG_APP_LOG_LEVEL**
+  Controls logging level for the main module.
+
+* **CONFIG_APP_MODULE_TRIGGER_TIMEOUT_SECONDS**
+  Default data sampling interval.
+
+* **CONFIG_APP_REQUEST_NETWORK_QUALITY**
+  When enabled, requests network quality metrics during regular sampling.
+
+* **CONFIG_APP_MSG_PROCESSING_TIMEOUT_SECONDS**
+  Maximum time allowed for processing a single message.
+
+* **CONFIG_APP_WATCHDOG_TIMEOUT_SECONDS**
+  Defines the watchdog timeout for the main module.
+
+
+## State Diagram
+
+The Main module implements a hierarchical state machine with the following states:
+
+```mermaid
+stateDiagram-v2
+    [*] --> STATE_RUNNING
+
+    state STATE_RUNNING {
+        [*] --> STATE_IDLE
+
+        STATE_IDLE --> STATE_TRIGGERING : CLOUD_CONNECTED_READY_TO_SEND
+        STATE_TRIGGERING --> STATE_IDLE : CLOUD_DISCONNECTED/CLOUD_CONNECTED_PAUSED
+
+        state STATE_TRIGGERING {
+            [*] --> STATE_SAMPLE_DATA
+            STATE_WAIT_FOR_TRIGGER --> STATE_SAMPLE_DATA : timer_trigger/button_press
+
+            STATE_SAMPLE_DATA --> STATE_WAIT_FOR_TRIGGER : LOCATION_SEARCH_DONE
+        }
+    }
+
+    STATE_RUNNING --> STATE_FOTA : FOTA_DOWNLOADING_UPDATE
+
+    state STATE_FOTA {
+        [*] --> STATE_FOTA_DOWNLOADING
+
+        STATE_FOTA_DOWNLOADING --> STATE_FOTA_WAITING_FOR_NETWORK_DISCONNECT : FOTA_SUCCESS_REBOOT_NEEDED
+        STATE_FOTA_DOWNLOADING --> STATE_FOTA_WAITING_FOR_NETWORK_DISCONNECT_TO_APPLY_IMAGE : FOTA_IMAGE_APPLY_NEEDED
+
+        STATE_FOTA_WAITING_FOR_NETWORK_DISCONNECT --> STATE_FOTA_REBOOTING : NETWORK_DISCONNECTED
+
+        STATE_FOTA_WAITING_FOR_NETWORK_DISCONNECT_TO_APPLY_IMAGE --> STATE_FOTA_APPLYING_IMAGE : NETWORK_DISCONNECTED
+        STATE_FOTA_APPLYING_IMAGE --> STATE_FOTA_REBOOTING : FOTA_SUCCESS_REBOOT_NEEDED
+
+        STATE_FOTA_REBOOTING --> [*] : sys_reboot()
+    }
 
-## State diagram
+    STATE_FOTA --> STATE_RUNNING : FOTA_DOWNLOAD_CANCELED/TIMED_OUT/FAILED
+```