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/mtest.rst b/doc/usage/cmd/mtest.rst
index 81d1f8f..e01f2a6 100644
--- a/doc/usage/cmd/mtest.rst
+++ b/doc/usage/cmd/mtest.rst
@@ -1,6 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2022, Heinrich Schuchardt <xypron.glpk@gmx.de>
 
+.. index::
+   single: mtest (command)
+
 mtest command
 =============