doc: describe command conitrace

Provide a man-page for the conitrace command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 0309abf..f75bd08 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -17,6 +17,7 @@
    bootefi
    bootmenu
    button
+   conitrace
    echo
    exit
    false