doc: man-page for bootz command

Provide a man-page for the bootz command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index c03f4ae..1c03b9b 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -30,6 +30,7 @@
    cmd/bootmenu
    cmd/bootmeth
    cmd/button
+   cmd/bootz
    cmd/cbsysinfo
    cmd/conitrace
    cmd/echo