Adds Stateful Set Basics as a tutorial

The content is in
docs/tutorials/stateful-applications/basic-stateful-set.md
The example application, named web, is in
docs/tutorials/stateful-applications/web.yaml
The tutorials TOC and index have been udpated to include a Stateful
Applications section of which this example is a subsection

_data/tutorials.yml

@@ -51,3 +51,7 @@ toc:
     path: /docs/tutorials/stateless-application/expose-external-ip-address-service/
   - title: Exposing an External IP Address to Access an Application in a Cluster
     path: /docs/tutorials/stateless-application/expose-external-ip-address/
+  title: Stateful Applications
+  section:
+  - title: Stateful Set Basics
+    path: /docs/tutorials/stateful-application/basic-stateful-set/