kubens: add page (#2617)

pages/common/kubens.md

@@ -0,0 +1,15 @@
+# kubens
+
+> Utility to switch between Kubernetes namespaces.
+
+- List the namespaces:
+
+`kubens`
+
+- Change the active namespace:
+
+`kubens {{name}}`
+
+- Switch to the previous namespace:
+
+`kubens -`