docs: rate limiting recipe + reference middleware

[SisyphusZheng]

Summary

Provide an official rate limiting recipe with documentation and a reference middleware implementation. No mainstream framework bundles rate limiting internally — all provide it as a recipe or middleware pattern.

Competitive context

Framework	Rate Limiting
Next.js	Recipe (Upstash/Redis in middleware.ts)
SvelteKit	User implements in hooks.server.js
Astro	User implements in onRequest middleware
Fresh	User implements in _middleware.ts
openElement	Zero documentation or templates

Proposed deliverable

1. Recipe doc (docs/integrations/rate-limiting.md):

   • In-memory sliding window (dev/single-instance)
   • Upstash Redis (production/multi-instance)
   • Deno KV (Deno Deploy)

2. Reference middleware (not a package — a documented pattern):

// app/middleware/rate-limit.ts
export function rateLimit(opts: { window: number; max: number }) {
  const store = new Map<string, { count: number; reset: number }>();
  return async (c, next) => {
    const key = c.req.header('x-forwarded-for') || 'unknown';
    // ... sliding window check
    if (exceeded) return c.text('Too Many Requests', 429);
    await next();
  };
}

3. Integration point: document how to attach via _middleware.ts (existing Hono middleware seam)

Non-goals

• No @openelement/rate-limit package
• No built-in Redis/KV dependency
• No per-route configuration API (use Hono middleware composition)

Target version

0.43.0 (documentation + recipe)