19791: Expand Python SDK examples into API client overview

19791: Expand Python SDK examples into API client overview

This change takes the skeleton of the existing examples page and expands
it into a fuller overview of the API client provided by the Python
SDK. It provides more explanation of how the client works; how it
corresponds to the Arvados API; more concrete examples of real tasks you
might actually want to do with the client; and a few different examples
to demonstrate how you might use different client code patterns for
different situations.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>