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    existing "maintained" examples and has been out-of-date for a while.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

_includes/templates/landing-page.md

@@ -1,121 +0,0 @@
-{% if page.cards %}<!-- check for this before going any further; if not present, skip to else at bottom -->
-<style>
-h2, h3, h4 {
-  border-bottom: 0px !important;
-  font-size: 22px !important;
-  padding-bottom: 20px !important;
-}
-.colContainer {
-  padding-top:2px;
-  padding-left: 2px;
-  overflow: auto;
-}
-#samples a {
-  color: #000;
-}
-.col3rd {
-  display: block;
-  float: left;
-  margin-right: 30px;
-  margin-bottom: 30px;
-  overflow: hidden;
-}
-.col3rd h3, .col2nd h3 {
-  margin-bottom: 0px !important;
-}
-.col3rd .button, .col2nd .button {
-  margin-top: 20px;
-  border-radius: 2px;
-}
-.col3rd p, .col2nd p {
-  margin-left: 2px;
-}
-.col2nd {
-  display: block;
-  width: 400px;
-  float: left;
-  margin-right: 30px;
-  margin-bottom: 30px;
-  overflow: hidden;
-}
-.shadowbox {
-  width: 250px;
-  display: inline;
-  float: left;
-  text-transform: none;
-  font-weight: bold;
-  text-overflow: ellipsis;
-  overflow: hidden;
-  line-height: 24px;
-  position: relative;
-  display: block;
-  cursor: pointer;
-  box-shadow: 0 2px 2px rgba(0,0,0,.24),0 0 2px rgba(0,0,0,.12);
-  border-radius: 5px;
-  background: #fff;
-  transition: all .3s;
-  padding: 16px;
-  margin: 0 16px 16px 0;
-  text-decoration: none;
-  letter-spacing: .01em;
-  height: 220px;
-}
-.shadowbox img {
-    min-width: 100px;
-    max-width: 100px;
-    max-height: 50px;
-    margin-right: 5px;
-    margin-bottom: 5px;
-    float: left;
-}
-</style>
-
-<div class="colContainer">
-{% for card in page.cards %}{% if card.title %}
-  <div class="col3rd shadowbox">
-    <h3>{{card.title}}</h3>
-    <p>{% if card.image %}<img src="{{card.image}}">{% endif %}{{card.description}}</p>
-  </div>
-{% endif %}{% endfor %}
-</div>
-
-{% else %}
-
-### ERROR: You must define "cards" front-matter YAML
-{: style="color:red" }
-
-This template requires that you insert YAML at the top of your document
-that defines the "cards" you'd like to display on the page. The cards will
-render in clickable boxes. 
-
-To get rid of this message and take advantage of this template, define `cards`:
-
-```yaml
----
-cards:
-- progression: no
-- card:
-  title: Mean Stack
-  image: /docs/meanstack/image_0.png
-  description: Lorem ipsum dolor it verberum.
-- card:
-  title: Guestbook + Redis
-  image: /images/docs/redis.svg
-  description: Lorem ipsum dolor it verberum.
-- card:
-  title: Cloud Native Cassandra
-  image: /images/docs/cassandra.svg
-  description: Lorem ipsum dolor it verberum.
-- card:
-  title: WordPress + MySQL
-  image: /images/docs/wordpress.svg
-  description: Lorem ipsum dolor it verberum. 
----
-```
-
-**Note:** If `progression` is set to `yes` then a "Start Here!" icon will be
-placed on the first card and arrows suggesting linear reading will be overlayed
-between the other cards, telling the reader that they should explore the content
-in a certain order. 
-
-{% endif %}