Leaf columns?

### Describe the usage question you have. Please include as many useful details as possible.

I'm running `cpp/examples/arrow/parquet_read_write.cc` with the values `x = {1, 3, 5, 7, 9}` and `y = {2, 4, 6, 8, 10}` and `chunk_size = 5`. I notice that the `Compress()` function is called once per column to write the page data and once per column when [`pager_->has_compressor()`](https://github.com/apache/arrow/blob/2d32efeedad88743dd635ff562c65e072cfb44f7/cpp/src/parquet/column_writer.cc#L970) I'm having trouble understanding what/why exactly the `Compress()` function is being called in the latter case.

### Component(s)

C++

Start with cpp/examples/arrow/parquet_read_write.cc and then inspect cpp/src/parquet/column_writer.cc around the referenced pager_->has_compressor() call. Trace the two Compress() call sites and document why the latter call occurs. Done means providing a clear explanation of the observed per-column behavior.

Written by the indexing model from the issue text.