Add polished documentation site and Alchemy deployment

[regenrek]

Summary

• add a first-class English Fumadocs site with installation, quickstart, lifecycle, concepts, guides, integrations, troubleshooting, operations, and complete CLI/MCP/HTTP references
• add generated-reference drift checks, documentation governance, release readiness, clean-install, browser, and OpenNext build verification
• configure Alchemy + OpenNext for Cloudflare Workers with the production custom domain docs.planr.so
• integrate the docs site into the monorepo, CI, release checks, and contributor guidance

Deployment contract

• production stage: prod
• custom domain: docs.planr.so
• worker: planr-docs-prod
• non-production stages do not claim the production hostname
• Cloudflare resources use Alchemy adoption for safe reconciliation

Verification

• pnpm peers check
• pnpm docs:content
• pnpm docs:typecheck
• pnpm docs:lint
• pnpm docs:verify-maintenance
• pnpm docs:verify-release
• pnpm docs:verify-deployment
• standard Next.js build: 62 pages
• Planr documentation goal: 26/26 acceptance checks