Add v5.3.0-alpha1 docs (#37662)

docs/5.3/examples/checkout/checkout.js

@@ -0,0 +1,19 @@
+// Example starter JavaScript for disabling form submissions if there are invalid fields
+(() => {
+  'use strict'
+
+  // Fetch all the forms we want to apply custom Bootstrap validation styles to
+  const forms = document.querySelectorAll('.needs-validation')
+
+  // Loop over them and prevent submission
+  Array.from(forms).forEach(form => {
+    form.addEventListener('submit', event => {
+      if (!form.checkValidity()) {
+        event.preventDefault()
+        event.stopPropagation()
+      }
+
+      form.classList.add('was-validated')
+    }, false)
+  })
+})()