Expand Guardian documentation: architecture page and integration tutorial

Source: DevEx Sync (Notion).

The Solutions section currently covers Guardians and private multisig at a high level, but Guardian docs need significant expansion: architecture deep-dive, integration tutorial(s), and end-to-end usage flows for app builders integrating Guardian-assisted self-custody.

Scope:

- Guardian architecture page (phase-2/phase-3 model, recovery flows, key topology).
- At least one Guardian integration tutorial under `/tutorials/`.
- Cross-link from Solutions → Tutorials and from Get Started where appropriate.

Upstream dependency: aligns with the OpenZeppelin Guardian SDK; coordinate framing with the Guardian product line.