docs: split README into separate docs

Move tech stack, getting started, and gameplay sections into docs/
folder. Slim README down to a landing page with links.

docs/getting-started.md

@@ -0,0 +1,25 @@
+# Getting Started
+
+## Prerequisites
+
+- [Node.js](https://nodejs.org/) (LTS recommended)
+- npm (comes with Node.js)
+
+## Setup
+
+```bash
+npm install
+npm run dev
+```
+
+Open the URL shown in the terminal (default: http://localhost:5173).
+
+## Scripts
+
+| Command | Description |
+|---------|-------------|
+| `npm run dev` | Start Vite dev server with HMR |
+| `npm run build` | Production build to `dist/` |
+| `npm run preview` | Preview the production build |
+| `npm test` | Run all tests once |
+| `npm run test:watch` | Run tests in watch mode |