nRF52840 Power Management - Phase 1 - Boot Low VBAT Voltage Lockout

Added NRF52840 power management core functionality:
- Boot‑voltage lockout
- Initial support for shutdown/reset reason storage and capture (via RESETREAS/GPREGRET2)
- LPCOMP wake (for voltage-driven shutdowns)
- VBUS wake (for voltage-driven shutdowns)
- Per-board shutdown handler for board-specific tasks
- Exposed CLI queries for power‑management status in CommonCLI.cpp
- Added documentation in docs/nrf52_power_management.md.
- Enabled power management support in Xiao nRF52840, RAK4631, Heltec T114 boards

variants/rak4631/RAK4631Board.h

@@ -30,6 +30,11 @@
 #define  ADC_MULTIPLIER   (3 * 1.73 * 1.187 * 1000)
 
 class RAK4631Board : public NRF52BoardDCDC, public NRF52BoardOTA {
+protected:
+#ifdef NRF52_POWER_MANAGEMENT
+  void initiateShutdown(uint8_t reason) override;
+#endif
+
 public:
   RAK4631Board() : NRF52BoardOTA("RAK4631_OTA") {}
   void begin();