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/size.rst b/doc/usage/cmd/size.rst
index f0c35e4..306fcba 100644
--- a/doc/usage/cmd/size.rst
+++ b/doc/usage/cmd/size.rst
@@ -1,5 +1,8 @@
 .. SPDX-License-Identifier: GPL-2.0+
 
+.. index::
+   single: size (command)
+
 size command
 ============