docs: Mintlify documentation site + API reference

[Goldokpa]

Adds a deployable Mintlify docs site under `docs-site/` — guides + a live API reference.

What's in it

• Guides: introduction, quickstart, core concepts, authentication & scopes, Python & TypeScript SDKs, production flow, MCP gateway/HITL, ACS Guardian, limits & errors, security.
• API Reference: auto-generated from the control plane's own OpenAPI schema (95 endpoints, 26 tags). Post-processed to add the production server URL (`https://api.identark.io\`) and a `csk_` bearer-auth scheme so the playground shows the Authorization field.
• Brand assets (logo light/dark, favicon), config (`docs.json`), and a `README.md` with run + spec-regeneration steps.

Verified locally with `mint dev` — both tabs, nav, theming, and the generated endpoint pages (method chip, Try-it, bearer auth, typed params, response examples) render.

To host at docs.identark.io

1. Connect `identark/identark` in the Mintlify dashboard; set content directory to `docs-site/` and the deploy branch to `main`.
2. Add the custom domain `docs.identark.io` in Mintlify → it gives a CNAME target.
3. Add that CNAME in Cloudflare (the `infrastructure/modules/dns` module).

Keeping the reference in sync

`docs-site/README.md` has a one-shot script to regenerate `api-reference/openapi.json` from the FastAPI app whenever the API changes.

🤖 Generated with Claude Code