doc: man-page for panic command

Provide a man-page for the panic 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 840c20c..ebf5eea 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -65,6 +65,7 @@
    cmd/md
    cmd/mmc
    cmd/mtest
+   cmd/panic
    cmd/part
    cmd/pause
    cmd/pinmux