doc: document sbi command

Add a man-page for the sbi command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 6def250..eedcdb1 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -19,3 +19,4 @@
    button
    mbr
    pstore
+   sbi