doc: man-page for mtest

Provide a man-page for the mtest command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 72c15ca..2c95471 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -63,6 +63,7 @@
    cmd/mbr
    cmd/md
    cmd/mmc
+   cmd/mtest
    cmd/part
    cmd/pause
    cmd/pinmux