Follow-up: document scale-to-zero landing page pattern

[justin808]

Finding

Post-merge audit of PR #347 found that a pre-merge maintainer request was not covered by the merged docs.

Evidence

• A maintainer comment on PR Docs: non-production cost optimization + Terraform/YAML guidance #347 asked to document the technique of using serverless scale-to-zero with a landing page while the app spins up.
• Later address-review summaries on PR Docs: non-production cost optimization + Terraform/YAML guidance #347 covered other docs fixes and reported no new actionable findings, but did not mention this landing-page/spin-up request.
• Current docs mention serverless scale-to-zero and cold-start tradeoffs, but do not document the landing-page pattern.

Desired outcome

Add docs for the scale-to-zero landing-page/spin-up pattern, ideally with enough guidance for demo/review/staging apps to route users to a lightweight landing page while a serverless app wakes.

Audit metadata

• Audit mode: completed-batch post-merge audit
• Audited PR: Docs: non-production cost optimization + Terraform/YAML guidance #347
• Audit ledger: not applicable (non-release audit)