Add release workflow and documentation

[martinabeleda]

Motivation

• Prepare ripdiff for automated crates.io releases with a maintainer workflow that keeps the crate version and git tag in sync.
• Move maintainer-only release procedures out of the user-facing README.md into a dedicated release guide.

Description

• Add a tag-driven GitHub Actions workflow at .github/workflows/release.yml that validates the pushed v* tag against Cargo.toml, runs cargo package, publishes with cargo publish --locked, and creates a GitHub release.
• Add release.toml to configure cargo-release as the maintainer entry point for releases: create vX.Y.Z tags, push to origin, and leave publishing to CI.
• Add RELEASING.md to document the release process, including the cargo-release workflow, the main-only release policy, and the required CARGO_REGISTRY_TOKEN secret.
• Update README.md to link to RELEASING.md instead of carrying maintainer release instructions inline.

Testing

• No additional code tests were run for this documentation and release configuration update.
• Verified that the documented cargo-release tag format (v{{version}}) matches the GitHub Actions trigger and tag/version validation logic.

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Codex Task