Add Messages API guide

[pelcasandra]

Summary

• add a bilingual guide for sending individual free-form and template messages through the Messages API
• explain technology, origin, customer profile, destination, attachments, WhatsApp service windows, asynchronous states, and duplicate-safe retries
• make the Developers overview channel-neutral and cross-link the new guide from the SMS and Templates API guides

Validation

• yarn build
• verified both generated EN/ES routes return HTTP 200 locally
• verified local links and anchors on both generated pages
• verified all referenced API anchors against the current API reference
• git diff --cached --check

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