docs(standard): keep _checks.yml per repo and record why the shared workflow was rejected

[lesnik512]

Summary

Follow-up to #95, closed as not planned. Sections 6 and 7 still described a shared checks.yml that "will live" in this repo with a run-time matrix. Now they describe what is actually the standard: a per-repo _checks.yml with a hand-maintained matrix, and why.

Changes

• Section 6: the matrix is a hand-maintained list in each repo's _checks.yml; a new Python is a one-line change per repo, made in one sweep.
• Section 7: the callers use the repo's own _checks.yml. The closing paragraph records the decision, so a reader does not re-propose it: the shared workflow was built and proven (Shared checks.yml in this repo with a derived Python matrix #95), and rejected because workflow_call inputs are scalars, which forces service containers and env maps into strings and image knowledge into the org repo.
• Section 8: release.yml no longer "stays" per repo in contrast to something shared; the sentence keeps the Trusted Publishing fact.

Checklist

• Lint and format pass (ruff) — Markdown only
• Type check passes (ty) — no Python changed
• Tests pass (uv run pytest: 125 passed, 29 skipped) and mkdocs build --strict is clean
• Build succeeds — no packaging change
• Docs updated — this is the docs change
• Repo metadata stays consistent across the three surfaces — not touched