feat(project): Restructure software for multi-app setup

- Reorganize the software directory to support multiple Zephyr applications (gateway, slave_node).

- Create a clear separation between applications and shared libraries.

- Add placeholder files for gateway and slave_node applications.

software/apps/slave_node/src/main.c

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2025 Eduard Iten
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+
+int main(void)
+{
+	printk("Hello from Slave Node!\n");
+	return 0;
+}