Add Facebook Messenger fundamentals guide

[pelcasandra]

Summary

• add a bilingual Facebook Messenger fundamentals guide covering Page connection, customer profiles, Inbox handling, the 24-hour messaging window, supported interactions, routes, compatible playbooks, pricing, reauthorization, and troubleshooting
• document current Messenger limitations in campaigns and custom AI agent incoming channels
• add the guide to Channels & Messaging navigation and cross-link it from setup, messaging, and Inbox documentation

Validation

• yarn build
• verified every internal link in both generated guide pages
• verified the guide appears in both channel category pages
• verified the English and Spanish guide routes return HTTP 200 locally

[netlify]

✅ Deploy Preview for legendary-lollipop-e2d131 ready!

Name	Link
🔨 Latest commit	bee9bfe
🔍 Latest deploy log	https://app.netlify.com/projects/legendary-lollipop-e2d131/deploys/6a767b0b8053680008306177
😎 Deploy Preview	https://deploy-preview-55--legendary-lollipop-e2d131.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.