docs: full prose for the five completion-gate validators

Found during v0.11.0-beta.1 release review (End User + PM). `docs/docs/data-engineering/validators.md` now has one-paragraph stubs for `dbt-build-green`, `dbt-nothing-built`, `dbt-deliverable-names`, `dbt-incremental-config`, `dbt-dialect-guard` (added in the release). Deferred: full reference sections (config knobs, false-positive notes, worked examples, the shadow-to-enforcement path) — a real writing task, not a 30-min patch. These are shadow-mode-only for now so a stub is sufficient for beta.

Read the existing one-paragraph stubs in docs/docs/data-engineering/validators.md for dbt-build-green, dbt-nothing-built, dbt-deliverable-names, dbt-incremental-config, and dbt-dialect-guard. Expand each with config knobs, false-positive notes, worked examples, and the shadow-to-enforcement path. Done means all five validators have full reference sections; they remain shadow-mode-only for the beta.

Written by the indexing model from the issue text.