[Docs] Add Cloudflare Workers AI provider setup guide

[q404365631]

Summary

Adds a dedicated Cloudflare Workers AI provider setup guide as requested in #38.

Changes

• New file: docs/providers/cloudflare.md — complete setup guide covering:
  • Prerequisites (Cloudflare account, Workers AI enabled)
  • Getting CLOUDFLARE_ACCOUNT_ID and CLOUDFLARE_API_KEY
  • Configuration via environment variables or web UI
  • Available models and model refresh
  • Troubleshooting common errors
• Updated README.md — added Provider Setup Guides section with link

Closes #38

[GoDiao]

Thanks for the guide! A couple of things to double-check before merging:

1. The default port in Free-Way is 8787, not 3000 — could you update the example URLs?
2. The model list in the guide might not match what's currently configured in the provider allowlist. You can check the actual models in src/providers/index.ts under the cloudflare section — it'd be great to list those instead so users aren't confused when they don't see the older models.

Otherwise the structure and troubleshooting sections look solid!

[GoDiao]

Hi @q404365631, just a friendly reminder about the two items from the review: 1) port should be 8787 instead of 3000, 2) the model list in the guide doesn't match the current allowlist. Let me know if you need any help!