Align doc slugs and add root meta.json for python/ and node/ #529

Same topic, different slug across SDKs:

• sub-workflows: py workflows/composition vs node/java workflows/sub-workflows
• autoscaling: py operations/autoscaler vs java operations/autoscaling
• email batching example: py examples/batch-emails vs node/java examples/bulk-emails
• events+webhooks: py combined extensibility/events-webhooks vs node/java split pages

Also: root meta.json exists only under java/ — add to python/ and node/ for explicit ordering. Found in the 2026-07-24 SDK parity audit (SDK_PARITY_REPORT.md).