[Python] Add example of how to set a schema field to a specific value

e.g.
```

fields = [
pa.field('foo', pa.int32()),
pa.field('bar', pa.string()),
pa.field('baz', pa.list_(pa.int8()))
]
s1 = pa.schema(fields)

s2 = s1.set(0, s1.field(0).with_type(pa.int64()))

```

See https://github.com/apache/arrow/issues/42012#event-17558114914 for more informaiton

No file or test is named in the issue. Start with the provided Python schema example and locate the cookbook section where schema operations are documented; add an example showing how to set a field to a specific value, then verify that the rendered documentation includes the example and matches the requested behavior.

Written by the indexing model from the issue text.