docs: add Openfort as a signer option

[emauja]

Summary

This PR adds documentation for using Openfort embedded wallets as a signer with ZeroDev Kernel accounts.

Changes

• Added docs/pages/sdk/signers/openfort.mdx - documentation for current SDK
• Added docs/pages/sdk/v5_3_x/signers/openfort.mdx - documentation for SDK v5.3.x
• Updated vocs.config.tsx to include Openfort in the sidebar under Auth Providers

About Openfort

Openfort is a wallet infrastructure platform that provides:

• Non-custodial embedded wallets - wallets are self-custodial by design
• Multiple authentication options - passkeys, social login, email
• EIP-1193 compatible provider - works seamlessly with viem, wagmi, and ethers
• Built-in ERC-4337 support - smart accounts with gas sponsorship, batching, and session keys

Integration Pattern

The integration follows the same pattern as other providers (Dynamic, Privy, Turnkey):

1. Initialize Openfort client
2. Authenticate user
3. Get EIP-1193 provider via openfort.embeddedWallet.getEthereumProvider()
4. Pass provider to ZeroDev's signerToEcdsaValidator

Testing

The code examples have been reviewed against Openfort's documentation to ensure accuracy.

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