doc: man-page for the wdt command

Describe the wdt command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 0aacf53..630a6cb 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -52,3 +52,4 @@
    size
    true
    ums
+   wdt