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/shell_system/shell_system.c

@@ -1,6 +1,25 @@
+/**
+ * @file shell_system.c
+ * @brief Provides basic system-level shell commands.
+ *
+ * This file implements essential system commands for the Zephyr shell,
+ * such as rebooting the device.
+ */
+
 #include <zephyr/shell/shell.h>
 #include <zephyr/sys/reboot.h>
 
+/**
+ * @brief Shell command to reset the system.
+ *
+ * This command performs a warm reboot of the device after a short delay
+ * to ensure the shell message is printed.
+ *
+ * @param sh The shell instance.
+ * @param argc Argument count.
+ * @param argv Argument values.
+ * @return 0 on success.
+ */
 static int cmd_reset(const struct shell *sh, size_t argc, char **argv)
 {
 	shell_print(sh, "Rebooting system...");