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

This reverts commit b2982a3e50.

diff/fd/diff.go

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