Add tempest.scenario.manager as stable interface for plugins

Implements: blueprint tempest-scenario-manager-stable
Change-Id: I5da35e4aafeb55b6754fd9cb86eeea04f869686e
diff --git a/doc/source/plugins/plugin.rst b/doc/source/plugins/plugin.rst
index 6726def..b1fd6f8 100644
--- a/doc/source/plugins/plugin.rst
+++ b/doc/source/plugins/plugin.rst
@@ -31,6 +31,7 @@
 * tempest.common.credentials_factory
 * tempest.clients
 * tempest.test
+* tempest.scenario.manager
 
 If there is an interface from Tempest that you need to rely on in your plugin
 which is not listed above, it likely needs to be migrated to tempest.lib. In