fix: add missing drag-drop examples (#616)

Adds some examples for the CDK `drag-drop` modules that weren't in the examples list.

Author: crisbeto

material.angular.io/src/app/shared/documentation-items/documentation-items.ts

@@ -428,14 +428,22 @@ const DOCS: {[key: string]: DocCategory[]} = {
           name: 'Drag and Drop',
           summary: 'Directives enabling drag-and-drop interactions',
           examples: [
-            'cdk-drag-drop-axis-lock',
-            'cdk-drag-drop-sorting',
             'cdk-drag-drop-overview',
-            'cdk-drag-drop-horizontal-sorting',
+            'cdk-drag-drop-axis-lock',
+            'cdk-drag-drop-boundary',
+            'cdk-drag-drop-connected-sorting',
+            'cdk-drag-drop-connected-sorting-group',
+            'cdk-drag-drop-custom-placeholder',
             'cdk-drag-drop-custom-preview',
-            'cdk-drag-drop-root-element',
+            'cdk-drag-drop-delay',
+            'cdk-drag-drop-disabled',
+            'cdk-drag-drop-disabled-sorting',
+            'cdk-drag-drop-enter-predicate',
+            'cdk-drag-drop-free-drag-position',
             'cdk-drag-drop-handle',
-            'cdk-drag-drop-connected-sorting',
+            'cdk-drag-drop-horizontal-sorting',
+            'cdk-drag-drop-root-element',
+            'cdk-drag-drop-sorting'
           ],
         },
         {