Document the new async feature in the Python SDK

[awalker4]

Making sure this is covered in the docs before we publish 0.26.0 with the new partition_async.

[Paul-Cornell]

It sounds like we now have two ways to use the SDK: one pattern is to use the SDK to process a single file synchronously; the other pattern is how to use the SDK to process multiple files asynchronously. If so, then we should probably lead with the the mutli-file async pattern, as this provides the most power and flexibility. We should then follow with the single file sync pattern--perhaps even noting that we don't prefer its use anymore but still available for backward compatibility (that said, there are some breaking changes with this way as well).

[Paul-Cornell]

(And we should probably update all of the Python SDK examples across the docs to now show how to use the multi-file async pattern only?)

[Paul-Cornell]

(A quick follow-up: we can wait until later to update all of the Python SDK examples across the docs to now show how to use the multi-file async pattern only.)