docs(next): chat UI with rsc example (#1481)

apps/next/package.json

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

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

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

apps/next/src/components/demo/chat/rsc/ai-action.tsx

apps/next/src/components/demo/chat/rsc/chat-section.tsx

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

apps/next/src/components/demo/chat/rsc/use-chat-rsc.tsx

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

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

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

pnpm-lock.yaml