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

Add copyright header to doc.go

Keep original comment style (e.g. line comments and block comments).

diff/fd/doc.go

@@ -0,0 +1,6 @@
+// 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"