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/for.rst b/doc/usage/cmd/for.rst
index f9e5049..4c98419 100644
--- a/doc/usage/cmd/for.rst
+++ b/doc/usage/cmd/for.rst
@@ -1,3 +1,6 @@
+.. index::
+   single: for (command)
+
 for command
 ===========