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/lib/CMakeLists.txt

@@ -0,0 +1,4 @@
+# Add your shared libraries here
+# Example:
+# add_library(modbus modbus/modbus.c)
+# target_include_directories(modbus PUBLIC .)