Refresh Mercado Libre integration guide

[pelcasandra]

Summary

• rewrite the Mercado Libre guide in English and Spanish around connection, synchronized data, Inbox workflows, order notes, and channel restrictions
• clarify that Mercado Libre messaging is tied to eligible orders and does not provide a complete historical customer import
• add Mercado Libre cross-links to the messaging and Inbox overviews without changing existing routes

Validation

• yarn build
• verified English and Spanish guide, messaging overview, and Inbox overview routes on the local Jekyll server

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