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/diff.go

@@ -2,7 +2,6 @@
 // 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"
 
 import (