add a small doc example showing ArrowWriter being used with a cursor

[msalib]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Now (as of version 15.0) that we can use std::io::Writer implementers with parquet::arrow::arrow_writer::ArrowWriter, we can just use std::io::Cursor instead of the now deleted InMemoryWriteableCursor. But there are no docs or examples of how to do that.

Describe the solution you'd like
I'd love to see just a tiny doc example showing that use case, something like this:

fn batch_to_bytes(batch: &RecordBatch) -> Result<Vec<u8>> {
    let mut buffer = std::io::Cursor::new(vec![]);
    let mut writer = ArrowWriter::try_new(&mut buffer, batch.schema(), None)?;
    writer.write(&batch)?;
    writer.close()?;
    Ok(buffer.into_inner())
}

[tustvold]

InMemoryWriteableCursor still exists it is just deprecated? You also can just use std::Vec directly, as suggested by the deprecation message?

fn batch_to_bytes(batch: &RecordBatch) -> Result<Vec<u8>> {
    let mut buffer = Vec::new();
    let mut writer = ArrowWriter::try_new(&mut buffer, batch.schema(), None)?;
    writer.write(&batch)?;
    writer.close()?;
    Ok(buffer)
}