Update docs for building datastore indexes. (#8707)

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

Author: Kami

packages/google-cloud-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