docs(governance): add deprecation policy for TeachLink Backend

[ghost-cy829]

Closes #1622

This PR adds Governance/policies/DEPRECATION.md, the general deprecation policy for TeachLink Backend, covering the deprecation notice period, the communication channels a deprecation must be announced through, and the criteria that must be met before something can actually be removed, as requested in the issue's implementation plan. It also adds one line to the Policies index in Governance/README.md linking to the new file, in the same style as the other entries there.

The repository already has a detailed domains/API_DEPRECATION.md that governs deprecation of REST endpoints, response fields, and SDK methods specifically. Rather than duplicate that, the new policy explicitly defers to it for API surfaces and instead covers everything else that the project exposes to contributors or other modules: configuration options and environment variables, internal modules and services used across module boundaries, documented scripts and CLI commands, and governance documents themselves.

This change touches exactly two files, and both are inside the Governance folder. No application code, no tests, and no CI configuration were changed.

On verification: every relative link in both files was checked against the current repository and resolves correctly. No regression test was added, because Jest's rootDir is set to src, which is outside the Governance-only scope this issue requires, and there is no existing test in the repo that checks the Governance folder's contents, so there was nothing applicable to update.

I was not able to run lint:ci, typecheck, or build locally. Running pnpm install --frozen-lockfile fails on a completely unmodified checkout of main with ERR_PNPM_LOCKFILE_CONFIG_MISMATCH, and I confirmed this by reproducing the same error with this change stashed out, so it is a pre-existing issue in the repository unrelated to this change. Since this repo's CI pipeline runs lint:ci, typecheck, and build in a clean environment on every PR, those checks will run there.

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