doc: usage: man-page for ums command

Provide a man-page for the ums command - USB Mass Storage.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index e5717b7..843b437 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -44,3 +44,4 @@
    scp03
    size
    true
+   ums