[ML DataFrame] Update Preview docs for source config change (#41761)

docs/reference/data-frames/apis/preview-transform.asciidoc

@@ -36,7 +36,9 @@ eCommerce sample data:
 --------------------------------------------------
 POST _data_frame/transforms/_preview
 {
-  "source": "kibana_sample_data_ecommerce",
+  "source": {
+    "index": "kibana_sample_data_ecommerce"
+  },
   "pivot": {
     "group_by": {
       "customer_id": {

docs/reference/rest-api/index.asciidoc

@@ -9,6 +9,7 @@ directly to configure and access {xpack} features.
 
 * <<info-api,Info API>>
 * <<ccr-apis,{ccr-cap} APIs>>
+* <<data-frame-apis,{dataframe-cap} APIs>>
 * <<graph-explore-api,Graph Explore API>>
 * <<freeze-index-api>>, <<unfreeze-index-api>>
 * <<index-lifecycle-management-api,Index lifecycle management APIs>>