Suggest improve emacs setting in 'The Perfect Setup'

[Feng Shu <tumashu@163.com>]

Hello:

In 'The Perfect Setup', we suggest user to use the below emacs config:

```
;; Assuming the Guix checkout is in ~/src/guix.
(with-eval-after-load 'geiser-guile
  (add-to-list 'geiser-guile-load-path "~/src/guix"))
```

This config need user to deal with %load-path of guix, which seem to be
a hard job for new guile user for many dirs of guix and other guix channels,
why not suggest new user to use the below config:

```
(setq geiser-guile-binary (list "guix" "repl"))
```

or:

```
(setq geiser-guile-binary (list "guix" "repl" "-L" "/path/to/user-own-config-dir"))
```

when user just want to develop his own guix config, just run:

```
emacs
```

guix and other channels installed by 'guix pull' will be used properly.

when user want to develop in guix.git, he just run emacs like the below:

```
./pre-inst-env emacs
```

The below is my guix emacs config, seem to work:

--------------------------------------------------------------------------------

(require 'geiser)
(require 'geiser-guile)
(require 'guix)
(require 'magit)

;; ** My own guix config.
(defvar eh-geeguix-dir (expand-file-name "~/geeguix"))

;; ** Let geiser-guile use 'guix repl'
(setq geiser-guile-binary (list "guix" "repl" "-L" eh-geeguix-dir))

;; ** guix-devel-mode
(add-hook 'scheme-mode-hook #'guix-devel-mode)

;; ** Get guix dir installed by 'guix pull'.
(defun eh-guix-dir ()
  (file-name-as-directory
   (cl-find-if
    (lambda (dir)
      (file-exists-p (expand-file-name "guix.scm" dir)))
    (directory-files
     (expand-file-name "~/.cache/guix/checkouts/")
     t))))

;; ** copyright-update
(when (eh-guix-dir)
  (let* ((dir (eh-guix-dir))
         (file (expand-file-name "etc/copyright.el" dir)))
    (when (file-exists-p file)
      (load-file file)
      ;; (add-hook 'after-save-hook 'copyright-update)
      (setq copyright-names-regexp
            (format "%s <%s>" user-full-name user-mail-address)))))

;; ** tempel 
(when (eh-guix-dir)
  (let* ((dir (eh-guix-dir))
         (path (expand-file-name "etc/snippets/tempel/*" dir)))
    (add-to-list 'tempel-path path)))



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