doc: hl78xx: add auto-baudrate detection and switching docs

Add detailed documentation for the HL78xx auto-baudrate feature,
including:

- AUTO_BAUDRATE_README.md: full description, use cases,
  configuration options, and troubleshooting
- QUICK_REFERENCE.md: concise reference for presets, Kconfig
  options, and supported baud rates

Covers:
- Feature overview and implementation details
- Supported baud rates and Kconfig options
- State machine integration
- Troubleshooting guidance
- Performance and optimization tips
- Integration examples

This improves usability and provides a quick reference for developers
working with the HL78xx modem in Zephyr projects.

Signed-off-by: Zafer SEN <zafersn93@gmail.com>

Author: zafersn