doc: sandbox: Add docs for the sb command

This command has a few small features, so document it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index b7c80ef..cb7a23f 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -104,6 +104,7 @@
    cmd/reset
    cmd/rng
    cmd/saves
+   cmd/sb
    cmd/sbi
    cmd/scmi
    cmd/scp03