feat(refactor): Restructure project for improved modularity and clarity

This commit introduces a major refactoring of the project structure to align
with Zephyr's recommended multi-application and library organization.

Key changes include:
- Relocation of custom modules from 'software/modules/' to 'software/lib/'.
- Introduction of a central 'software/CMakeLists.txt' to manage application
  and library subdirectories.
- Creation of new Kconfig files for 'software/' and 'software/apps/slave_node/'
  to define project-wide and application-specific configurations.
- Removal of the 'gateway' and 'stm32g431_tests' applications.
- Removal of 'shell_modbus.c' and 'shell_system.c' from 'slave_node' application's
  direct source files, indicating a shift towards library-based shell commands.
- Updates to 'software/apps/slave_node/CMakeLists.txt', 'prj.conf', and
  'boards/bluepill_f103rb.conf' to reflect the new structure and dependencies.

software/apps/slave_node/src/main.c

@@ -1,9 +1,9 @@
 #include <zephyr/kernel.h>
 #include <zephyr/settings/settings.h>
 #include <zephyr/logging/log.h>
-#include <modbus_server.h>
-#include <valve.h>
-#include <fwu.h>
+#include <lib/modbus_server.h>
+#include <lib/valve.h>
+#include <lib/fwu.h>
 
 LOG_MODULE_REGISTER(main, LOG_LEVEL_INF);