docs(deploy): spec + explanation for set_replica_identity (#1447)

[dimitri-yatsenko]

Summary

Adds two new pages documenting the `dj.deploy.set_replica_identity` helper that lands in DataJoint 2.3 (datajoint-python #1466). Closes the docs side of datajoint-python #1447.

File	Role
`src/reference/specs/deploy-operations.md` (new)	Normative spec for the new `datajoint.deploy` module + the `set_replica_identity` function. Includes a Design rationale section explaining the three structural choices (migration-only; not in `dj.migrate`; new module for an emerging category).
`src/explanation/postgresql-cdc-replication.md` (new)	Conceptual explainer: what `REPLICA IDENTITY` is, why CDC consumers care (Lakehouse Sync mandates `FULL` and silently skips tables that lack it), cost and compliance considerations, and the representative workflow.
`mkdocs.yaml`	New "Operations" group under Concepts; new "Deployment" group under Reference → Specifications.

The spec carries the formal API contract; the explainer carries the reasoning and the WAL/compliance tradeoffs that motivate the feature. Both pages cross-link to each other.

Why a new module page rather than folding into existing specs

`datajoint.deploy` is the first of an emerging category of operational helpers (publication membership, vacuum/reindex, role grants are plausible siblings). Giving it a dedicated spec page now — with one inhabitant — establishes the boundary against `datajoint.migrate` and provides a home for future helpers without retroactive reorganization. The rationale section in the spec walks through the alternatives that were rejected.

Sequencing

This PR is independently reviewable but should land alongside or after datajoint-python #1466 — the code that implements the function. If they merge in either order, no broken links result; both stand alone.

Test plan

• `mkdocs serve` renders both new pages under their new nav groups
• Cross-links resolve (spec ↔ explainer, both → PostgreSQL docs)
• No `mkdocs build --strict` warnings introduced
• Re-read both pages once datajoint-python #1466 lands to confirm the API matches the shipped code

[MilagrosMarin]

Verified carefully against datajoint-python#1466:

✅ Signature, return shape, error messages match impl exactly
✅ Schema.list_tables() already excludes ~/~~ tables (schemas.py:525-545) — the CDC-relevant filter
✅ Cross-links resolve; nav placement under Concepts → Operations and Reference → Specifications → Deployment is sensible
✅ The "Design rationale" section is unusually well-argued — the migration-only argument (mixed-state failure mode from a config-flag + utility combo) is the right framing
✅ Databricks Lakehouse Sync "silently skipped" framing accurately motivates the feature

Sequencing note: impl PR #1466 is still open (I requested changes on the assert usage). This docs PR is independently reviewable but ideally lands alongside or after the impl.

Approving — the spec and explainer are a clean pair.