[SPARK-40027][PYTHON][SS][DOCS] Add self-contained examples for pyspa…

…rk.sql.streaming.readwriter

### 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.

Closes #37461 from HyukjinKwon/SPARK-40027.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>