docs(extensions/nanoarrow_device): Document device extension

As I was reminded in https://github.com/apache/arrow-nanoarrow/pull/488#discussion_r1617989935 , we have very minimal top-level documentation for the nanoarrow device extension (we do have inline function documentation, though). After some of the larger-scale details are sorted out, the high-level documentation and/or a getting started article should be added (which will also help flush out the scope of the extension and how it might be used).

Start by reading the nanoarrow device extension's existing inline function documentation and the discussion in pull request 488. Once the larger-scale details are settled, define the high-level documentation or getting-started article around the extension's scope and intended use. Done means the extension has useful top-level guidance for new users.

Written by the indexing model from the issue text.