docs: add mixed encoding mode documentation

[yeqown]

Summary

• Add comprehensive documentation for QR Code mixed encoding mode
• Include both English and Chinese versions
• Document mode indicators, data structure, encoding algorithm, and optimization considerations

Changes

• docs/mixed-encoding-mode.md: New documentation file covering:
  • Overview and benefits of mixed encoding
  • Mode indicators (Numeric, Alphanumeric, Byte, Kanji, ECI)
  • Data structure for each segment
  • Encoding example with 日本語123
  • Step-by-step encoding algorithm
  • Optimization considerations for mode switching
  • Current implementation status and future plans

Related

• Part of the Kanji encoding mode implementation effort
• Documents the theoretical foundation for future mixed mode implementation

Test plan

• Documentation follows the same format as kanji-encoding.md
• Both English and Chinese versions are complete and accurate