doc: loadx man-page

Provide a man-page for the loadx command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index f348848..84ef8a9 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -65,6 +65,7 @@
    cmd/load
    cmd/loadb
    cmd/loadm
+   cmd/loadx
    cmd/loady
    cmd/mbr
    cmd/md