docs: add chapter-to-module quickstart index

[however-yir]

Summary

Add a chapter-to-module quickstart index to the root README.

Why

This repository covers many Spring AI topics, but new readers currently have to infer where each chapter or capability lives in the codebase. A small index makes the project easier to navigate and lowers the onboarding cost.

Scope

• add a Quickstart Index section to README.md
• map the main topics to their backend modules and frontend entry points
• keep the index short and focused on discovery

Testing

• documentation-only changes
• reviewed the listed module paths against the current repository structure