Add doc example of constructing a MapArray

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
People on a discord channel were recently confused about creating MapArrays

The current documentation https://docs.rs/arrow/41.0.0/arrow/array/struct.MapArray.html does not give an example of how to create MapArrays

Describe the solution you'd like
A clear example of how to construct a MapArray linked from the MapArray docs

Describe alternatives you've considered

Additional context
@tustvold suggests "using the builder" which I think means https://docs.rs/arrow/41.0.0/arrow/array/struct.MapBuilder.html

Here is the example code for constructing it:

fn build_map_array(list: Vec<HashMap<String, proto::Value>>) -> Result<Arc<MapArray>> {
    let mut entry_offset = vec![];
    let mut keys = vec![];
    let mut values = vec![];

    let mut offset = 0;
    for kv in list {
        entry_offset.push(offset as u64);
        offset += kv.len();

        for (key, value) in kv {
            keys.push(key);
            values.push(format!("{}:{}", value.type_name(), value));
        }
    }
    println!("offset list: {:?}", entry_offset);

    let map_data = ArrayDataBuilder::new(datatype_map())
        .len(3)
        .add_buffer(Buffer::from_vec(entry_offset))
        .add_child_data(
            StructArray::from(vec![
                (
                    Arc::new(Field::new("keys", DataType::Utf8, false)),
                    Arc::new(StringArray::from(keys)) as ArrayRef,
                ),
                (
                    Arc::new(Field::new("values", DataType::Utf8, false)),
                    Arc::new(StringArray::from(values)) as ArrayRef,
                ),
            ])
            .into_data(),
        )
        .build()?;

    Ok(Arc::new(MapArray::from(map_data)))
}

(which I am not sure is right)

[alamb]

label_issue.py automatically added labels {'arrow'} from #4382