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/armffa.rst b/doc/usage/cmd/armffa.rst
index 13fa90c..4f41e33 100644
--- a/doc/usage/cmd/armffa.rst
+++ b/doc/usage/cmd/armffa.rst
@@ -1,6 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2022-2023 Arm Limited and/or its affiliates <open-source-office@arm.com>
 
+.. index::
+   single: armffa (command)
+
 armffa command
 ==============