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).

Fix doc comments containing multiple line 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 (