feat(docs): add Persona-powered documentation assistant

[omarkawach]

Summary

Initial addition of a Persona-powered assistant to the WebMCP documentation site. The assistant can answer documentation questions and use read-only WebMCP page tools through a Cloudflare Worker proxy.

Changes

• Added and configured the Persona documentation widget.
• Registered a read-only WebMCP tool for accessing the current documentation page.
• Added a Cloudflare Worker proxy for Persona chat requests.
• Stored the Persona API key in Cloudflare Secrets Store and attached it through a Worker binding.
• Added local development, deployment, type generation, and typecheck scripts.
• Added Wrangler and TypeScript configuration for the proxy.
• Added .dev.vars.example and ignored local secrets.
• Added .mintignore to prevent Worker source and configuration from being published by Mintlify.
• Added the required dependencies and workspace catalog entries.

Related Issues

None.

Checklist

• PR title follows conventional commit format: feat(docs): add Persona-powered documentation assistant
• I followed docs/AI_CONTRIBUTION_MANIFESTO.md for safety, contracts, and SSOT decisions
• I followed docs/MCPB_PACKAGE_PHILOSOPHY.md for package boundaries and ownership (no package boundaries changed)
• I followed docs/TESTING_PHILOSOPHY.md and selected the right test layer(s) (docs validation, typecheck, and deployment validation)
• Code builds without errors (pnpm build from apps/documentation-website)
• Tests pass (no applicable unit tests for this docs integration)
• Linting passes (vp check currently reports formatting in wrangler.jsonc)
• TypeScript compiles (pnpm run typecheck:persona-proxy)
• Changeset added (not applicable because the documentation website is private and no package is being released)

Testing

Ran the following focused checks:

cd apps/documentation-website
pnpm build
pnpm run types:persona-proxy
pnpm run typecheck:persona-proxy
vp exec wrangler deploy --dry-run --config wrangler.jsonc