Add an examples layout.

Reduces duplication and makes maintenance easier.

site/docs/4.1/examples/checkout/form-validation.js

@@ -0,0 +1,20 @@
+// Example starter JavaScript for disabling form submissions if there are invalid fields
+(function () {
+  'use strict'
+
+  window.addEventListener('load', function () {
+    // Fetch all the forms we want to apply custom Bootstrap validation styles to
+    var forms = document.getElementsByClassName('needs-validation')
+
+    // Loop over them and prevent submission
+    Array.prototype.filter.call(forms, function (form) {
+      form.addEventListener('submit', function (event) {
+        if (form.checkValidity() === false) {
+          event.preventDefault()
+          event.stopPropagation()
+        }
+        form.classList.add('was-validated')
+      }, false)
+    })
+  }, false)
+}())