Add Simplified Chinese README translation

[whtis]

What

Adds README.zh-CN.md, a Simplified Chinese translation of the project README, and a small language switcher in the original README.md so readers can jump between the two.

Why

A large share of PageIndex's audience — long-document RAG users in finance, legal, regulatory and academic settings — reads Chinese as a first language. Several recent issues and Discord questions are in Chinese, and a localized README lowers the barrier to evaluating PageIndex for self-host vs. cloud, and to running the cookbooks. The translation preserves the original structure, headings, callouts, and link targets exactly, so future README updates can be mirrored line-by-line with minimal effort.

A few translation choices worth flagging:

• Vectorless → 无需向量 / 无向量, depending on whether it modifies a noun or a clause, to keep the prose natural.
• Vector DB / Chunking → 向量数据库 / 切块, which match the conventions used in the major Chinese AI/ML communities (e.g. LangChain-Chatchat, LLaMA-Factory docs).
• Agentic, vectorless RAG → 智能体式、无向量 RAG, keeping the RAG acronym untranslated since Chinese readers in this space already use it as-is.
• The "vibe retrieval" aside is rendered as 凭感觉、靠近似的"玄学式"向量检索, which reads naturally in Chinese while preserving the slightly tongue-in-cheek tone of the original.

How tested

• README.zh-CN.md rendered locally via a Markdown previewer; all images, badges, links, anchors and <details> blocks render identically to the English version.
• Verified that no link targets were changed (every https://, every examples/... and cookbook/... relative path matches the English README).
• Cross-checked the language switcher on both files navigates correctly.
• No code/config changes; nothing in the package, examples, or CI touched.

Drafted with AI assistance; verified by author before publishing.