chore: mention Arcana Auth in a few places

[mmjee]

Very minor.

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)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
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PR-Codex overview

Detailed summary

• Added "Arcana Auth" as an example of a signer in the document "site/smart-accounts/signers/choosing-a-signer.md"
• Added a link to the document "site/smart-accounts/signers/choosing-a-signer" in the document "site/overview/why-account-kit.md"

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