docs: update warm runtime configuration after overlay fixes

[ak684]

Summary

• documents merge-by-name behavior between installer-managed and API-managed warm runtime configurations
• moves Runtime API password setup to the durable Replicated Admin Console field
• removes the legacy takeover and default re-declaration workflow
• documents source attribution, deletion fallback, and post-upgrade handling
• includes migration guidance for installs that already saved v1_current as a legacy workaround

Stack

• Base documentation PR: docs: Multiple custom sandbox images with warm runtime pools #609
• Runtime API fixes: https://github.com/OpenHands/runtime-api/pull/730 and https://github.com/OpenHands/runtime-api/pull/731
• OHE chart fixes: fix: rename warm-runtime default config to v1_current to match the app default spec lookup OpenHands-Cloud#1181, fix: give the Runtime API admin password a real KOTS field with a generated default OpenHands-Cloud#1182, and feat: enable warm-runtime config overlay mode for Replicated OpenHands-Cloud#1183

Merge gate

Keep this PR in draft until:

1. The base documentation PR is merged.
2. Runtime API publishes a release newer than 0.9.0 containing docs: add Agent Canvas visuals for weekly review 2026-08-18 #730 and automation: Agent Canvas documentation image requests — 2026-08-18 #731.
3. An OHE release pins that Runtime API release and includes the related chart changes.
4. The requirements section is updated with the exact first supported OHE version.

OpenHands-Cloud main currently still pins runtime-api 0.9.0, so the new behavior is not customer-shipped yet.

Validation

• git diff --check
• Mintlify broken-link scan: no new failures; it reports three existing unrelated broken links in cloud-api.mdx, agent-server.mdx, and local-server.mdx