docs(governance): add release checklist

[Sammydz456]

closes #1620

Summary

Adds a reusable release checklist for TeachLink Backend and links it from the Governance template index.

Changes

• Covers pre-release validation, required sign-offs, production deployment, and post-release follow-up.
• References existing governance policies for environment promotion, migrations, configuration, SBOMs, and hotfixes.
• Documents why no dedicated regression test applies to this documentation-only change.

Validation

• Workspace diagnostics passed.
• Local checklist links and git diff --check passed.
• Changes are limited to two files under Governance.

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