docs: enhance LX200CommandSet.md with OpenAstroTech extensions

Updated command reference to include comprehensive OpenAstroTech firmware
extensions alongside the standard Meade LX200 protocol specification.

Changes:
- Added OAT column to command support matrix
- Documented 50+ OAT-specific X-family commands
- Added Appendix B: OpenAstroTech Extensions with detailed specs
- Added Appendix C: OAT vs Meade behavioral differences
- Documented OAT enhancements to standard commands (GPS, Movement, Focus)
- Included Hall sensor homing, digital level, backlash compensation
- Added mount status queries, stepper control, and configuration commands

Based on OpenAstroTech wiki documentation (Firmware V1.13.12).

This provides a comprehensive reference for both standard Meade LX200
compatibility and OAT-specific advanced features.

Author: andre-stefanov