Add GPT-5.6 model pricing docs

[ai-bankofai]

Summary

• Add GPT-5.6 Sol, Terra, and Luna model docs in English and Chinese
• Add GPT-5.6 pricing rows to English and Chinese pricing tables
• Update pricing notes for GPT-5.6 cache-write and long-context rules
• Add GPT-5.6 entries to English and Chinese LLM sidebars

Source of truth

• GitLab apenft/bai-documents

Verification

• yarn build