doc: add man-page for cmp command

Provide a man-page for the cmp command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 815b032..d6283fa 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -35,6 +35,7 @@
    cmd/cat
    cmd/cbsysinfo
    cmd/cls
+   cmd/cmp
    cmd/conitrace
    cmd/cyclic
    cmd/dm