docs: migrate validation docs to Lit (#1673)

Author: crutchcorn

docs/config.json

@@ -234,6 +234,10 @@
               "label": "Basic Concepts",
               "to": "framework/lit/guides/basic-concepts"
             },
+            {
+              "label": "Form Validation",
+              "to": "framework/lit/guides/validation"
+            },
             {
               "label": "Arrays",
               "to": "framework/lit/guides/arrays"