Add missing S3 extension to Server Extensions overview

[Community-Programmer]

This PR adds the missing “S3” extension entry to the Server Extensions overview table.

The S3 extension already exists in the documentation sidebar and has a full page, but the overview list did not include it, creating an inconsistency. This update adds:

• S3 - “Persist documents to Amazon S3 or any S3-compatible storage service.”

This keeps the overview consistent with the rest of the extensions (Database, Logger, Redis, SQLite, Throttle, Webhook).

This is a documentation-only change.

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Related issues: N/A

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