Add documentation for running codegen
Add documentation for running codegen

This documentation provides a model for creating new Python Vkit
projects and running code generation in them, based on how we're
currently doing it for Bigtable.

Change-Id: I237a8525846a2d4e664a38fd667f9e4c5c7d155d

geigerj/gax-python,googleapis/gax-python,geigerj/gax-python