doc: unbind man-page

Provide a man-page for the unbind command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 54ef89e..9553278 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -97,6 +97,7 @@
    cmd/trace
    cmd/true
    cmd/ums
+   cmd/unbind
    cmd/ut
    cmd/wdt
    cmd/wget