doc: man-page for itest

Provide a man-page for the itest command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 66d73e7..2f211f7 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -72,6 +72,7 @@
    cmd/history
    cmd/host
    cmd/if
+   cmd/itest
    cmd/imxtract
    cmd/load
    cmd/loadb