feat: enhance docs chat with vector-based RAG pipeline

- Added vector index building and serving capabilities to the docs chat.
- Introduced new scripts for generating embeddings and serving the chat API using vector search.
- Updated package.json with new commands for vector index operations.
- Enhanced README with instructions for the new RAG pipeline and legacy keyword pipeline.
- Removed outdated Vercel configuration file.

scripts/docs-chat/run.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+: "${OPENAI_API_KEY:?OPENAI_API_KEY is required}"
+
+pnpm docs:chat:index:vector
+pnpm docs:chat:serve:vector