docs: finalize multi-image warm runtime configuration

[ak684]

Summary

• documents multiple custom sandbox images with warm runtime pools for OpenHands Enterprise 0.64.0 or later
• explains merge-by-name behavior for installer-managed and API-managed configurations
• documents durable Runtime API admin password setup, source attribution, deletion fallback, and sandbox selector constraints

Related PRs

• Base documentation PR: docs: Multiple custom sandbox images with warm runtime pools #609
• Runtime API implementation: https://github.com/OpenHands/runtime-api/pull/730 and https://github.com/OpenHands/runtime-api/pull/731
• OHE chart integration: 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

Release

OpenHands Enterprise 0.64.0 is available in the Stable channel as Replicated sequence 1597. It pins enterprise-server 1.59.1, runtime-api 0.10.0, and agent-server 1.46.0.

Merge order

1. Review and merge this PR into docs/warm-runtime-custom-images.
2. Review the resulting complete guide in docs: Multiple custom sandbox images with warm runtime pools #609.
3. Merge docs: Multiple custom sandbox images with warm runtime pools #609 into main after its checks pass.

Validation

• runtime-api 0.10.0 targeted tests: 30 passed
• OHE 0.64.0 release guards: 2 passed
• OHE 0.64.0 runtime-api Helm tests: 3 passed
• git diff --check: passed
• 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