feat: add example using chat-ui components to docs (#1436)

.changeset/fresh-dancers-refuse.md

apps/next/package.json

apps/next/src/app/api/chat/route.ts

apps/next/src/components/demo/chat.tsx

apps/next/src/content/docs/llamaindex/guide/chat.mdx

apps/next/src/content/docs/llamaindex/guide/meta.json

apps/next/tailwind.config.js

pnpm-lock.yaml