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.