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

@@ -57,3 +57,5 @@ toc:
     path: /docs/tutorials/stateful-application/run-stateful-application/
   - title: Running a Replicated Stateful Application
     path: /docs/tutorials/stateful-application/run-replicated-stateful-application/
+  - title: Stateful Set Basics
+    path: /docs/tutorials/stateful-application/basic-stateful-set/