docs: Add PDB sensor documentation for liquid-cooled systems (SN58XX_LD)

- Add 16 new API sections for PDB (Power Distribution Board) sensors
  - PDB Hotswap Controller sensors (current, voltage, power, temperature)
  - PDB Power Converter sensors (current, voltage, power, temperature)
  - PDB MOSFET Ambient temperature sensor
  - PDB hot-plug events and alarm statuses
- Create generic CHANGELOG_User_Manual.md for tracking documentation updates
- Add system tree reference from SN5810_LD platform for verification
- Update Section 3.1.11 with liquid-cooled system clarification
- All APIs verified against actual hardware and code implementation
- Target platforms: SN5810_LD (HI181), SN5800_LD (HI182)
- Release: V.7.0050.3000

Author: acoifmannvidia