docs(release): add verification guide

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Summary

• add a release verification guide for checksums, chart signatures, image signatures, SBOM attestations, GitHub provenance, and install/upgrade/rollback smoke
• link the guide from install and security docs and MkDocs navigation
• bump release metadata and install examples to v0.5.7

Closes #266

Tests

• bash -n scripts/verify-install.sh
• go test ./api/v1alpha2 -run 'TestPreStableReleaseTrainMarkersStayDocumented|TestReleaseVersionMarkersStayInSync'\n- scripts/validate-yaml-json\n- VERSION=v0.5.7 make release-smoke\n- python3 scripts/check-markdown-links.py\n\n## Not run\n- mkdocs build -f docs/mkdocs.yml (mkdocs is not installed in this workspace)\n

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