Add deprecated docs warning mechanism (#4731)

* Deprecated docs warning

* Unknown tag 'endcomment'

* only add css when necessary

* try lighter bgcolor

* tweak copy

* change development url to point at vnext-staging

* try callout

* rollback

* change var names

* update var used

* fix var typo

* turn off deprecation message

_includes/deprecation-warning.html

@@ -0,0 +1,12 @@
+{% if page.deprecated %}
+<section id="deprecationWarning">
+  <main>
+    <div class="content deprecation-warning">
+      <h3>
+        Documentation for Kubernetes {{ page.version}} is no longer actively maintained. The version you are currently viewing is a static snapshot.
+        For up-to-date documentation, see the <a href="{{ page.currentUrl }}">latest</a> version.
+      </h3>
+    </div>
+  </main>
+</section>
+{% endif %}

_includes/head.html

@@ -8,6 +8,7 @@
     <link rel="stylesheet" type="text/css" href="/css/jquery-ui.min.css">
     <link rel="stylesheet" type="text/css" href="/css/sweetalert.css">
     <link rel="stylesheet" type="text/css" href="/css/callouts.css">
+    {% if page.deprecated %}<link rel="stylesheet" type="text/css" href="/css/deprecation-warning.css">{% endif %}
     {% if page.class == "gridPage" %}<link rel="stylesheet" type="text/css" href="/css/gridpage.css">{% endif %}
     {% if page.css %}<link rel="stylesheet" type="text/css" href="{{ page.css }}"><!-- custom css added -->{% else %}<!-- no custom css detected -->{% endif %}