Clean up `docs/snippets.py` file

[busunkim96]

This file should be moved into the samples directory samples/snippets or removed entirely. It is used to add code to usage.rst but is not tested at the moment.

https://cloud.google.com/logging/docs/samples

[daniel-sanche]

This was addressed in #140. The snippets have been moved to samples/snippets/usage_guide.py tests have been added