firestore: samples should include client creation

Currently, we have [one sample for creating a client](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/4282aa905763af06354225ae0f422c09201b41da/firestore/cloud-async-client/snippets.py#L27), and the others [assume the client already exists](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/4282aa905763af06354225ae0f422c09201b41da/firestore/cloud-async-client/snippets.py#L36).

This causes problems, because we have both a sync and an async client, and using the wrong client leads to errors. When the samples are viewed stand-alone, they don't give the proper context

We should instead include the creation of the client (and any important imports) as part of each sample, so they can better stand alone