Rewrite watermark explanation, add realtime producer, rename jobs

- Reframe watermark as "when to publish" instead of "when to drop"
- Add Mermaid diagrams: late event before/after publishing
- Move watermark from pass-through job to aggregation section
- Rename event_watermark -> event_timestamp, start_job -> pass_through_job
- Add producer_realtime.py with simulated delays and NYC zone annotations
- Add aggregation_job_demo.py with 10-second windows for experimentation
- Remove Step N: prefixes from section headings
- Change watermark interval to 5 seconds