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/bdinfo.rst b/doc/usage/cmd/bdinfo.rst
index 5261085..a21fbc8 100644
--- a/doc/usage/cmd/bdinfo.rst
+++ b/doc/usage/cmd/bdinfo.rst
@@ -1,6 +1,9 @@
 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2023, Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
 
+.. index::
+   single: bdinfo (command)
+
 bdinfo command
 ==============