Stabilize docs on Cloudflare Workers

[regenrek]

Why

• Cloudflare Workers Free limits requests to 10 ms of CPU, while Fumadocs/Next.js viewport prefetches generated avoidable RSC traffic.
• OpenNext generated SSG cache entries, but the Alchemy deployment path did not populate them after deployment.

How

• Disable automatic prefetching centrally for Fumadocs links and explicitly on landing-page links.
• Enable OpenNext's static-assets incremental cache with cache interception.
• Make pnpm docs:deploy populate the remote SSG cache after the Alchemy production deployment and document the contract.

Tests

• pnpm --filter @planr/docs typecheck
• targeted ESLint for changed source files
• pnpm --filter @planr/docs verify:maintenance
• pnpm --filter @planr/docs verify:release (55 routes, 121 links, 38 redirects)
• pnpm --filter @planr/docs build
• production Alchemy deploy and OpenNext remote cache population completed successfully

Known platform boundary

Cloudflare Workers Free still enforces a hard 10 ms CPU limit. The changes reduce avoidable Worker execution and serve prerendered content from the supported OpenNext cache path; Workers Paid remains the fully deterministic option for heavier Next.js execution.