Add doc/RELEASING.md and reference it from POLICY.md and README.md

[id774]

Motivation

• Provide a clear, maintainer-focused runbook for building, inspecting and publishing the automatic gem so releases are consistent and auditable.
• Surface the release procedure from top-level docs and the policy so maintainers know the manual steps and the immutability/verification requirements for published versions.

Description

• Add a comprehensive doc/RELEASING.md that documents prerequisites, authentication, build and inspection steps, smoke tests, tagging and publication guidance for manual gem releases.
• Update doc/POLICY.md packaging section to require green CI for releases, to state that published versions are immutable, and to point to the manual publication procedure in RELEASING.md.
• Update README.md to include a link to doc/RELEASING.md in the repository documents table.

Testing

• This change is documentation-only; no automated tests were run as part of this PR and no code behavior was modified.

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Codex Task