/docs/samples revamp: move examples to /tutorials

Addresses the work described at #4134.

- Remove /docs/samples page: This page mostly had links to examples on
  kubernetes/kubernetes (examples dir) which is now at kubernetes/examples
  repository. It was linking to README.md files for tutorials. Now removing
  this page as we are moving tutorials from kubernetes/examples to here:

  - Create redirects from /docs/samples/ to /docs/tutorials/ (both JS and
    redirect_from).
  - Deprecate "MEAN Stack" tutorial: This tutorial overlaps quite a bit with
    existing "maintained" examples and has been out-of-date for a while.
    Creating an external redirect to its original and more up-to-date location:
    https://medium.com/google-cloud/running-a-mean-stack-on-google-cloud-platform-with-app-engine-and-mongolab-4bbd2040ea75

- Import README.md files of maintained examples (guestbook, wordpress+mysql and
  cassandra) to this repository and serve them under /docs/tutorial.

- Update /docs/tutorials ToC:
  - Link to imported tutorials.
  - Fix missing articles on sidebar (_data/tutorials.yml) and make the tutorials
    listed on tutorials/index.md available on the sidebar (closes #4007).

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

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