docs: add developer reference for Web3AuthMPCCoreKit API, including constructor, methods, and status details

[ieow]

Motivation and Context

Jira Link:

Description

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project. (run lint)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code requires a db migration.

---

Note

Low Risk
Documentation-only change; no runtime or security behavior is modified.

Overview
Adds docs/developer.md, a new developer reference for the public Web3AuthMPCCoreKit API (types in src/interfaces.ts).

The doc covers constructor options (storage, session, OAuth, TSS/DKG, hashed factors), COREKIT_STATUS, and method-by-method guidance for init/rehydration, JWT and OAuth login, factor/MFA flows (inputFactorKey, enableMFA, createFactor/deleteFactor, manualSync), signing (including makeEthereumSigner), session/account helpers, _UNSAFE_* export APIs, and helpers such as TssSecurityQuestion—with parameter tables and TypeScript examples throughout.

^{Reviewed by Cursor Bugbot for commit a3b4210. Bugbot is set up for automated code reviews on this repo. Configure here.}