Update docs for building datastore indexes. (#8707)

Restore `index.yml` file removed in PR #3146.

CONTRIBUTING.rst

@@ -200,7 +200,7 @@ Running System Tests
    > --key-file=${GOOGLE_APPLICATION_CREDENTIALS}
 
    # Create the indexes
-   $ gcloud datastore create-indexes system_tests/data/index.yaml
+   $ gcloud datastore indexes create datastore/tests/system/index.yaml
 
 - For datastore query tests, you'll need stored data in your dataset.
   To populate this data, run::

datastore/tests/system/index.yaml

@@ -0,0 +1,23 @@
+indexes:
+
+- kind: Character
+  ancestor: yes
+  properties:
+  - name: appearances
+
+- kind: Character
+  ancestor: yes
+  properties:
+  - name: alive
+
+- kind: Character
+  ancestor: yes
+  properties:
+  - name: family
+  - name: appearances
+
+- kind: Character
+  ancestor: yes
+  properties:
+  - name: name
+  - name: family