doc: sleep man-page

Provide a man-page for the sleep command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 7d4a1cb..72c15ca 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -76,6 +76,7 @@
    cmd/scp03
    cmd/setexpr
    cmd/size
+   cmd/sleep
    cmd/sound
    cmd/source
    cmd/temperature