Add rate limiting documentation and tests

[jeremydmiller]

Summary

• Add rate-limited endpoint using ASP.NET Core's [EnableRateLimiting] attribute
• Configure fixed window rate limiter in sample app (1 req/10s, 429 rejection)
• Add integration tests verifying 429 response when limit exceeded
• Add documentation page explaining rate limiting with Wolverine.HTTP (uses standard ASP.NET Core middleware)

Test plan

• 2 integration tests pass (rate limited returns 429, non-limited always succeeds)

Closes #2404

🤖 Generated with Claude Code