Figures and associated text in Windowing section of programming guide should be updated

All figures and associated text in [section 8.1 of the programming guide](https://beam.apache.org/documentation/programming-guide/#windowing-basics) should be updated and clarified to be more intuitive (i.e. not sourcing from a database table, changing "Table rows" to some other intermediate PCollection, etc.).

Also, Figure 4 in 8.1.2 uses KafkaIO, but the text in that section refers to TextIO and describes windowing with a bounded collection, so the figure should be updated to match the text and scenario described.

Imported from Jira [BEAM-9998](https://issues.apache.org/jira/browse/BEAM-9998). Original Jira may contain additional context.
Reported by: dwrede.