Document the release process

[kitsuyui]

Summary

• Add release process documentation that defines the release artifact, version tag policy, release criteria, checklist, pre-release handling, and failed-release handling.
• Link the release process from the README documentation list.

Checks

• git diff --check
• typos README.md docs/release.md action.yml .github/workflows/*.yml
• npx --yes markdownlint-cli2 docs/release.md
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.12 -color
• collateral check: clean

Notes

• The full README currently has pre-existing MD013 line-length warnings, so markdownlint was run against the new release document.
• The pull request GitHub Actions matrix is the source of truth for cross-platform action execution.