docs: Refresh getting started docs

[athreesh]

Summary

• Adds clearer README entry points with a crisp Start Here path for new users and links out to deeper docs.
• Adds docs/COMPATIBILITY.md for support status and tested/runtime pins across ModelExpress, vLLM, Dynamo, SGLang, TRT-LLM, and platform dependencies.
• Adds root AGENTS.md in a SGLang-style structure, focused on user-facing MX help: what MX does, how to route users by goal, deployment questions, metadata backend guidance, key env vars, and debugging playbooks.
• Aligns Helm values, Helm docs, and public example manifests to the current 0.5.0 ModelExpress server image.
• Fixes public-doc correctness issues: closes a broken deployment Markdown fence, removes internal Artifactory/runner details from deployment docs, replaces the broken DCO troubleshooting link, and removes a placeholder Helm repo URL.

Validation

• git diff --check
• Markdown fence scan across AGENTS/README/docs/examples/Helm docs
• stale 0.3.0 reference scan across AGENTS, README, docs, helm, examples, .github, and ci
• helm lint helm

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