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/askenv.rst b/doc/usage/cmd/askenv.rst
index b85cefa..e2b3c53 100644
--- a/doc/usage/cmd/askenv.rst
+++ b/doc/usage/cmd/askenv.rst
@@ -1,5 +1,8 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
+.. index::
+   single: askenv (command)
+
 askenv command
 ==============