Add a versioning policy for TeachLink Backend - #1731

Close: #1618

Created Governance/policies/VERSIONING.md — the only file changed, all inside Governance/.
Contents:

• SemVer 2.0.0 scheme with the current 0.0.1 state documented and 1.0.0 as the stability declaration
• Bump criteria for each level: MAJOR (removed/renamed GraphQL fields, removed REST endpoints, narrowed DTO types, tightened validation, changed error shapes, raised Node/Postgres minimums), MINOR (additive fields/endpoints/optional env keys/enum values only where consumers tolerate unknowns), PATCH (bug fixes, refactors, docs)
• Pre-release conventions: alpha/beta/rc/dev identifiers, monotonic counters, the rc.N → release promotion gate, hotfix versioning from the release line
• Plus build metadata, a two-release deprecation window with a fix exemption, expand/contract rules for migrations, and the release process
  Notable callout documented: tightening validation (e.g. making an @IsOptional() DTO field required) is a MAJOR change even though nothing is deleted — the most commonly missed trigger.