docs: align monitoring docs for postgresai 0.15

Summary

Release-readiness cleanup for postgresai 0.15 monitoring docs.

Changes

• Pin CLI examples to postgresai@0.15.0 instead of @latest.
• Replace stale monitoring CLI flags with the 0.15 commands and options.
• Point metric-definition links at the 0.15.0 tag.
• Align local retention/resource guidance with the 0.15 compose defaults.
• Patch remaining stale CLI/Docker install references after review: monitoring user default, local-install help output, pgwatch service names, and Docker troubleshooting examples.

Validation

• Searched monitoring docs for stale @latest, 0.14.0, old CLI flags, and old metric links. Remaining 90d references are custom examples or Helm-specific values, not local-install defaults.
• bun run build passed locally after installing deps with bun install --frozen-lockfile; only pre-existing blog author warnings were emitted.
• Pipeline passed: https://gitlab.com/postgres-ai/docs/-/pipelines/2561262859

Closes #221 (closed)