doc: create index entries for commands

Add the description of CLI commands to the generated index.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/cmd/extension.rst b/doc/usage/cmd/extension.rst
index 6366cf5..4c261e7 100644
--- a/doc/usage/cmd/extension.rst
+++ b/doc/usage/cmd/extension.rst
@@ -1,6 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2021, Kory Maincent <kory.maincent@bootlin.com>
 
+.. index::
+   single: extension (command)
+
 extension command
 =================