SenseCAP Solar repeater: add poweroff/shutdown support and NRF52 power-management parity

[Specter242]

Problem

SenseCAP Solar repeater firmware currently lacks a practical software power-off path and no CLI poweroff/shutdown command.

On battery-powered deployments this makes maintenance harder and does not match expected NRF52 power-management behavior.

Repro

1. Build/flash SenseCap_Solar_repeater.
2. Connect over serial CLI.
3. Try poweroff or shutdown.

Current behavior

• Command is not available.
• Device cannot be cleanly put into low-power/system-off from CLI.

Expected behavior

• poweroff and shutdown CLI commands are available.
• SenseCAP Solar board implementation enters system-off cleanly, with wake configured from user button.
• Variant-level configuration aligns with NRF52 power-management support used by other devices.

Proposed fix

• Add SenseCAP Solar board power-management implementation (initiateShutdown, powerOff, button/wake handling).
• Enable NRF52_POWER_MANAGEMENT and required variant defines.
• Add CLI aliases poweroff and shutdown in common CLI command handling.
• Update docs matrix for SenseCAP Solar support.

Related: #1145