[3.2.x] Added Google Cloud Spanner to list of third-party DB backends.

Backport of 125f9af from main

Author: vi3k6i5

docs/ref/databases.txt

@@ -1067,6 +1067,7 @@ by 3rd parties that allow you to use other databases with Django:
 
 * `CockroachDB`_
 * `Firebird`_
+* `Google Cloud Spanner`_
 * `Microsoft SQL Server`_
 
 The Django versions and ORM features supported by these unofficial backends
@@ -1076,4 +1077,5 @@ the support channels provided by each 3rd party project.
 
 .. _CockroachDB: https://pypi.org/project/django-cockroachdb/
 .. _Firebird: https://pypi.org/project/django-firebird/
+.. _Google Cloud Spanner: https://pypi.org/project/django-google-spanner/
 .. _Microsoft SQL Server: https://pypi.org/project/django-mssql-backend/