Formalize dbt producer coverage matrix

[PragnyaKhandelwal]

Summary

• Add a formal coverage matrix for dbt producer tests (versions, facets, scenarios).
• Document baseline expectations and current gaps.
• Link the policy from the dbt producer README.

Context

This PR makes the dbt compatibility coverage expectations explicit so regressions are easier to catch and support windows are clear.

Changes

• Add coverage matrix documentation.
• Reference it in the dbt producer README.

Testing

• Not run (docs-only change).

Notes / Follow-ups

• Add scenarios for incremental, test, snapshot coverage.
• Expand matrix to multiple dbt-core minor versions if agreed by maintainers.