docs: add key technologies page for RAG and LLM Wiki paradigms

[jasen215]

Summary

• Add docs/site/ecosystem/key-technologies.md (EN) and zh-CN/ecosystem/key-technologies.md (ZH)
• Covers both paradigms side by side:
  • RAG technologies: document parsing, chunking strategies (9 types), embedding models, vector databases, retrieval strategies, query optimization, reranking, orchestration frameworks, evaluation
  • LLM Wiki technologies: knowledge compilation (incl. STORM/Co-STORM), knowledge representation, FTS/graph indexing, quality maintenance, MCP interface, file conversion pipeline
• Includes a paradigm comparison table at the bottom (10 dimensions)
• Add "Key Technologies" item to Ecosystem sidebar (EN + ZH)

Sources

• Network search (RAG tech stack 2025)
• WikiLoop MCP knowledge base (Karpathy LLM Wiki paradigm, Sage Wiki, STORM)
• RAG architecture panorama source-note

Test plan

• Local pnpm docs:build passes
• Confirm https://jasen215.github.io/wikiloop/ecosystem/key-technologies is accessible after merge