docs: flesh out minimal user guide

We want to post a minimal user guide at `google-cloud-swift.github.io`, which we will then migrate to docs.cloud.google.com. The user guide should include:

Put into /guides

Begin by inspecting the repository for the proposed /guides location and any existing documentation or site entry points. Before editing, clarify the guide's required contents and migration expectations; done means a minimal user guide is defined and published at google-cloud-swift.github.io in a form ready to migrate to docs.cloud.google.com.

Written by the indexing model from the issue text.