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/rng.rst b/doc/usage/cmd/rng.rst
index 1a352da..274e4d8 100644
--- a/doc/usage/cmd/rng.rst
+++ b/doc/usage/cmd/rng.rst
@@ -1,5 +1,8 @@
 .. SPDX-License-Identifier: GPL-2.0+
 
+.. index::
+   single: rng (command)
+
 rng command
 ===========