[SPARK-40027][PYTHON][SS][DOCS] Add self-contained examples for pyspark.sql.streaming.readwriter

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to improve the examples in pyspark.sql.streaming.readwriter by making each example self-contained with a brief explanation and a bit more realistic example.

Why are the changes needed?

To make the documentation more readable and able to copy and paste directly in PySpark shell.

Does this PR introduce any user-facing change?

Yes, it changes the documentation

How was this patch tested?

Manually ran each doctest.

[HyukjinKwon]

cc @viirya and @HeartSaVioR mind taking a look please when you find some time?

[HyukjinKwon]

either 1) you've tested these examples manually or 2) these examples will be automatically tested via CI?

Both, yes :-).

[HyukjinKwon]

Merged to master.

[HyukjinKwon]

I am touching these examples a lot. So all posthoc reviews are very appreciated!