Add buttons to outlining and tweak `C-h b' buffer

Add buttons to outlining and tweak `C-h b' buffer

* lisp/help.el (describe-bindings-outline): Change default to t.
(describe-bindings): Tweak the look and remove the explanation.

* lisp/outline.el (outline-minor-mode-use-buttons):
(outline-minor-mode-buttons): New user options.
(outline-hide-subtree): Update buttons.
(outline--make-button, outline--make-button-overlay)
(outline--insert-open-button, outline--insert-close-button)
(outline--fix-up-all-buttons): New functions.