docs(aio): move links from nav groups to items

Fundamentals and Techniques nav groups we also links to pages. This caused
counterintuitive behaviour when clicking on them.

This commit moves each link from the group item to a children item, called
Introduction.

Closes angular#16604

aio/content/navigation.json

@@ -87,10 +87,15 @@
 
     {
       "title": "Fundamentals",
-      "url": "guide/fundamentals",
       "tooltip": "The fundamentals of Angular",
       "children": [
 
+        {
+          "url": "guide/fundamentals",
+          "title": "Introduction",
+          "tooltip": "An introduction to the fundamentals of Angular."
+        },
+
         {
           "url": "guide/animations",
           "title": "Animations",
@@ -259,10 +264,15 @@
 
     {
       "title": "Techniques",
-      "url": "guide/techniques",
       "tooltip": "Techniques for putting Angular to work in your environment",
       "children": [
 
+        {
+          "url": "guide/techniques",
+          "title": "Introduction",
+          "tooltip": "An introduction to techniques to use when developing with Angular."
+        },
+
         {
           "url": "guide/security",
           "title": "Security",