Add Openfort signer guide to Smart Accounts Kit docs

### Description

The [Configure signers](https://docs.metamask.io/smart-accounts-kit/guides/smart-accounts/signers/) section of the Smart Accounts Kit docs has guides for using third-party embedded wallet providers (Privy, Dynamic) as smart account signers. I'd like to add an equivalent guide for [Openfort](https://www.openfort.io).

Openfort provides embedded wallets with social/email/passkey sign-in via the [`@openfort/react`](https://www.npmjs.com/package/@openfort/react) SDK. Configured with `accountType: EOA`, the embedded wallet works as a signer for MetaMask Smart Accounts through the standard Wagmi Wallet Client — the same pattern as the existing Privy guide.

### Proposed change

- New page `smart-accounts-kit/guides/smart-accounts/signers/openfort.md`, following the structure of `privy.md`: prerequisites, install, provider setup (Openfort + Wagmi), then `toMetaMaskSmartAccount` with the Wagmi Wallet Client as signer.
- Add Openfort to the signers index card list and to `gator-sidebar.js`.

Disclosure: I'm a founder at Openfort. Happy to write the PR and keep the guide up to date — assigning myself per the contribution workflow (or please assign me, since external contributors can't self-assign).

Start by reading smart-accounts-kit/guides/smart-accounts/signers/privy.md and the existing signers index card list. Add the Openfort guide at smart-accounts-kit/guides/smart-accounts/signers/openfort.md, then update gator-sidebar.js and the index so the new guide is discoverable. Done means the page covers prerequisites, installation, Openfort and Wagmi provider setup, and the signer pattern described in the issue.

Written by the indexing model from the issue text.