Recommended file extension for Feather files

Python cookbook uses `.feather` (https://github.com/apache/arrow-cookbook/blob/90cdc3c5516f475149f172f869a1538cc50e8e68/python/source/io.rst#L535)

Arrow docs suggest `.arrow` instead of `.feather` https://arrow.apache.org/docs/format/Columnar.html#ipc-file-format

Which is it?

Compare the Python cookbook example in python/source/io.rst around the Feather file example with the linked Arrow IPC file-format documentation. Confirm the recommended extension and any distinction between the names, then update the cookbook wording so the recommendation is consistent and explicit.

Written by the indexing model from the issue text.