Document the dolt_ci_workflow_* system tables #68

Fourteen system tables related to Dolt CI workflows are registered in the canonical system-table registry at doltdb/system_table.go but have no documentation in reference/sql/version-control/dolt-system-tables.md:

• dolt_ci_workflows
• dolt_ci_workflow_events
• dolt_ci_workflow_event_triggers
• dolt_ci_workflow_event_trigger_branches
• dolt_ci_workflow_event_trigger_activities
• dolt_ci_workflow_jobs
• dolt_ci_workflow_steps
• dolt_ci_workflow_saved_query_steps
• dolt_ci_workflow_saved_query_step_expected_row_column_results
• dolt_ci_workflow_dolt_test_steps
• dolt_ci_workflow_dolt_test_step_groups
• dolt_ci_workflow_dolt_test_step_tests

This is a related family of tables, so probably wants either a new sub-section in dolt-system-tables.md ("CI workflow tables") or a dedicated page under products/dolthub/continuous-integration/. Each table needs a schema + brief description. Add the resulting anchors to sql-extensions.md once documented.

Surfaced during the source-vs-docs cross-check for #43 (see #64).