Update signers.mdx to add Para

[adeets-22]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
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PR-Codex overview

This PR updates the documentation for third-party signers by adding a new entry that highlights the integration of Para with the Alchemy AA SDK, including a link to setup instructions.

Detailed summary

• Added a new entry to the documentation for Para with Alchemy AA SDK.
• Included a link to setup instructions for using Para with Alchemy.

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