/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    existing "maintained" examples and has been out-of-date for a while.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>

docs/tutorials/index.md

@@ -1,5 +1,8 @@
 ---
 title: Tutorials
+redirect_from:
+- "/docs/samples/"
+- "/docs/samples.html"
 ---
 
 This section of the Kubernetes documentation contains tutorials.
@@ -17,6 +20,8 @@ each of which has a sequence of steps.
 
 * [Running a Stateless Application Using a Deployment](/docs/tutorials/stateless-application/run-stateless-application-deployment/)
 
+* [Example: PHP Guestbook application with Redis](/docs/tutorials/stateless-application/guestbook/)
+
 * [Using a Service to Access an Application in a Cluster](/docs/tutorials/stateless-application/expose-external-ip-address-service/)
 
 * [Exposing an External IP Address to Access an Application in a Cluster](/docs/tutorials/stateless-application/expose-external-ip-address/)
@@ -29,6 +34,10 @@ each of which has a sequence of steps.
 
 * [Running a Replicated Stateful Application](/docs/tutorials/stateful-application/run-replicated-stateful-application/)
 
+* [Example: WordPress and MySQL with Persistent Volumes](/docs/tutorials/stateful-application/mysql-wordpress-persistent-volume/)
+
+* [Example: Deploying Cassandra with Stateful Sets](/docs/tutorials/stateful-application/cassandra/)
+
 * [Running ZooKeeper, A CP Distributed System](/docs/tutorials/stateful-application/zookeeper/)
 
 #### Connecting Applications