Remove unnecessary demo container
Remove unnecessary demo container

- Previously instructed users to build demo container via doc/Dockerfile.
- Since rendering isn't working in the notebook and the rest of the installed dependencies are now available in the base tensorflow-notebook container building a container isn't necessary.
- Users are now instructed to run the base tensorflow-notebook-cpu container and clone the example code with git.
- The git clone command refers to https://github.com/kubeflow/examples instead of the URL of this fork so the docs will be incorrect in that regard until this is merged into master. Optionally until then we can add an instruction to switch branch.

kubeflow/examples,kubeflow/examples,kubeflow/examples,kubeflow/examples