bug#56260: 29.0.50; Provide a way to define shortdoc groups without loading shortdoc.el

[daanturo]

Currently to use `define-short-documentation-group`, we must first load
`shortdoc.el`, but loading it isn't exactly a cheap task: my startup is
slowed down by ~0.0186s.

We can achieve this with an autoloaded progn:

```elisp

;; Allow registering shortdoc groups without loading this file

;;;###autoload
(progn
  (defvar shortdoc--groups nil)

  (defmacro define-short-documentation-group (group &rest functions)
    ;; docstring
    (declare (indent defun))
    `(progn
       (setq shortdoc--groups (delq (assq ',group shortdoc--groups)
                                    shortdoc--groups))
       (push (cons ',group ',functions) shortdoc--groups))))

```

-- 
Daanturo.