docs: fix documentation structure (#949)

Co-authored-by: Astha Mohta <35952883+asthamohta@users.noreply.github.com>

Author: dandhlee

docs/api-reference.rst

@@ -1,5 +1,5 @@
-Spanner Client
-==============
+Spanner Client Usage
+====================
 
 .. _spanner-client:
 

docs/client-usage.rst

@@ -1,5 +1,5 @@
-Database Admin
-==============
+Database Admin Usage
+====================
 
 After creating an :class:`~google.cloud.spanner_v1.instance.Instance`, you can
 interact with individual databases for that instance.

docs/database-usage.rst

@@ -5,27 +5,48 @@
 Usage Documentation
 -------------------
 .. toctree::
-  :maxdepth: 1
-  :titlesonly:
+  :maxdepth: 2
 
   client-usage
-  instance-usage
-  database-usage
   table-usage
   batch-usage
   snapshot-usage
   transaction-usage
 
+  database-usage
+  instance-usage
+
 API Documentation
 -----------------
 .. toctree::
   :maxdepth: 1
   :titlesonly:
 
-  api-reference
   advanced-session-pool-topics
   opentelemetry-tracing
 
+  spanner_v1/client
+  spanner_v1/instance
+  spanner_v1/database
+  spanner_v1/table
+  spanner_v1/session
+  spanner_v1/keyset
+  spanner_v1/snapshot
+  spanner_v1/batch
+  spanner_v1/transaction
+  spanner_v1/streamed
+
+  spanner_v1/services
+  spanner_v1/types
+  spanner_admin_database_v1/services
+  spanner_admin_database_v1/types
+  spanner_admin_database_v1/database_admin
+  spanner_admin_instance_v1/services
+  spanner_admin_instance_v1/types
+  spanner_admin_instance_v1/instance_admin
+
+
+
 Changelog
 ---------
 

docs/index.rst

@@ -1,5 +1,5 @@
-Instance Admin
-==============
+Instance Admin Usage
+====================
 
 After creating a :class:`~google.cloud.spanner_v1.client.Client`, you can
 interact with individual instances for a project.

docs/instance-usage.rst

@@ -1,5 +1,5 @@
-Table Admin
-===========
+Table Admin Usage
+=================
 
 After creating an :class:`~google.cloud.spanner_v1.database.Database`, you can
 interact with individual tables for that instance.