docs: Add Doxygen comments to library files

Added Doxygen-style comments to all C source and header files in the
 and  directories. This improves
code documentation and enables VSCode tooltip help.

Additionally, short inline comments were added to all global variables
for better clarity.

software/lib/valve/valve.c

@@ -1,3 +1,12 @@
+/**
+ * @file valve.c
+ * @brief Implementation of the motorized valve control library.
+ *
+ * This file contains the logic for controlling a motorized valve using a
+ * VND7050AJ high-side driver. It uses a delayed work item to handle the
+ * safety timeouts for opening and closing operations.
+ */
+
 #include <zephyr/kernel.h>
 #include <zephyr/settings/settings.h>
 #include <zephyr/logging/log.h>
@@ -20,8 +29,16 @@ static enum valve_state current_state = VALVE_STATE_CLOSED;
 static enum valve_movement current_movement = VALVE_MOVEMENT_IDLE;
 static uint16_t max_opening_time_s = 60;
 static uint16_t max_closing_time_s = 60;
-static struct k_work_delayable valve_work;
+static struct k_work_delayable valve_work; // Work item for scheduling valve movement timeouts
 
+/**
+ * @brief Work handler for valve movement timeouts.
+ *
+ * This function is executed when the valve's movement timer expires.
+ * It stops the motor to prevent damage and updates the valve's state.
+ * 
+ * @param work Pointer to the k_work item.
+ */
 static void valve_work_handler(struct k_work *work)
 {
     gpio_pin_set_dt(&valve_gpios.in0, 0);