all: use fixdocs tool to store package documentation in doc.go

Add copyright header to doc.go

diff/fd/doc.go

@@ -0,0 +1,8 @@
+// Copyright ©2017 The gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package fd provides functions to approximate derivatives using finite differences.
+*/
+package fd // import "gonum.org/v1/gonum/diff/fd"