docs: add hardware wiring guide for common sensor setups

## Description
Create visual wiring guides for connecting common sensors to ESP32.

## Requirements
- [ ] DHT22 (temperature/humidity) wiring diagram
- [ ] DS18B20 (waterproof temperature) wiring diagram
- [ ] PIR motion sensor wiring diagram
- [ ] Relay module wiring diagram
- [ ] Pin reference table

## Skills
Markdown, electronics, ESP32