What this PR does / why we need it:

The development guide repeats universal_online_stores for both offline and online store selection. Document the existing universal_offline_stores(only=["bigquery"]) and universal_online_stores(only=["sqlite"]) markers and explain that they select separate dimensions of the integration test matrix.

Which issue(s) this PR fixes:

No linked issue.

Checks

• I've made sure the targeted documentation validation is passing.
• My commits are signed off (git commit -s).
• My PR title follows conventional commits format.

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Validated against current master (b8989cb0033b0d17248ab62ed839cf93cb5d1159). Executed the checkout's actual pytest_generate_tests with configuration and service boundaries stubbed. Verified the offline bigquery mapping to the GCP provider and the online sqlite mapping to the local provider. Inspected both marker mappings and ran git diff --check. No cloud services or full integration matrix were run.

Misc

Preparation was AI-assisted; the wording was checked against current parametrization code. Please apply the existing kind/housekeeping label and authorize external-contributor CI with ok-to-test if needed.

NONE