doc: man-page for imxtract

Provide a man-page for the imxtract command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 84ef8a9..54ef89e 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -62,6 +62,7 @@
    cmd/fwu_mdata
    cmd/gpio
    cmd/host
+   cmd/imxtract
    cmd/load
    cmd/loadb
    cmd/loadm