docs: cover all callback stages when splitting a monolithic pipeline

[joshua-temple]

Problem

The monolith-decomposition guidance only gave actionable detail for build and deploy, but cascade calls four user callback stages - validate, build, deploy, publish - plus an optional changelog callback. The migration table had no validate or publish row, so a team splitting a single monolithic pipeline (lint/test, build, deploy, and release-tag fused in one pass) got no guidance for the validate and publish seams.

Fix

• Add validate (lint/test job -> validate callback) and publish (release-tag/retag step -> publish callback at the release boundary) rows to the migration table, ordered as the full validate -> build -> deploy -> publish sequence.
• Rename the subsection to "Splitting a monolithic pipeline" and broaden it: a monolith usually fuses lint/test, build, deploy, and release-tag, so adoption means teasing them apart along every seam cascade calls across. Build/deploy stays the load-bearing split (build once, promote the same SHA-pinned artifact, never rebuild per stage).
• Clarify that orchestrate/promote/release/rollback/hotfix and version/changelog derivation are cascade-owned generated workflows, not user callbacks. Repoint all in-page anchors after the rename.

Verification

• Docs-only change.
• Independent review: APPROVE. All callback inputs verified against the Callback Contract; no lingering references to the old anchor; table renders.