docs: replace what-is-a-signer with previously longer doc

[Dan-Nolan]

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

This PR updates the documentation for Alchemy Signer, introducing a structured overview of criteria for selecting signers, types of signers, and supported signers, enhancing clarity and detail for users.

Detailed summary

• Added a new section titled Criteria to consider with key factors for choosing a signer.
• Introduced detailed descriptions for different custody and security models.
• Listed various authentication methods supported by Alchemy Signer.
• Explained types of signers: non-custodial wallets, MPC wallets, decentralized MPC networks, self-custodial wallets, and custodial wallets.
• Provided examples for each type of signer.
• Clarified the SmartAccountSigner and SmartAccountAuthenticator interfaces and their implementations.

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