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/lib/shell_system/CMakeLists.txt

@@ -0,0 +1 @@
+zephyr_library_sources(shell_system.c)

software/lib/shell_system/Kconfig

@@ -0,0 +1,5 @@
+config SHELL_SYSTEM
+    bool "Enable Shell System"
+    default y
+    help
+      Enable the system commands.

software/lib/shell_system/shell_system.c

@@ -0,0 +1,12 @@
+#include <zephyr/shell/shell.h>
+#include <zephyr/sys/reboot.h>
+
+static int cmd_reset(const struct shell *sh, size_t argc, char **argv)
+{
+	shell_print(sh, "Rebooting system...");
+	k_sleep(K_MSEC(100)); // Allow the shell to print the message
+	sys_reboot(SYS_REBOOT_WARM);
+	return 0;
+}
+
+SHELL_CMD_REGISTER(reset, NULL, "Reboot the system", cmd_reset);