Unify outline-minor-mode-use-buttons with in-margins/insert values (bug#57813)

Unify outline-minor-mode-use-buttons with in-margins/insert values (bug#57813)

* doc/emacs/text.texi (Outline Mode): Remove outline-minor-mode-use-margins.
Document the values insert/in-margins of outline-minor-mode-use-buttons.

* lisp/help.el (describe-bindings):
Set outline-minor-mode-use-buttons to 'insert'.

* lisp/textmodes/emacs-news-mode.el (emacs-news--mode-common):
Set outline-minor-mode-use-buttons to 'in-margins'.

* lisp/outline.el (outline-minor-mode-use-buttons): Change :type
from 'buffer-predicate' to choice of const values nil/in-margins/t.
(outline--use-buttons, outline-minor-mode-insert-buttons)
(outline-minor-mode-use-margins, outline--use-margins): Remove variables.
(outline-minor-mode-highlight-buffer): Change overlay name
'outline-overlay' to 'outline-highlight'.
(outline-minor-mode): Simplify to handle possible values of
'outline-minor-mode-use-buttons' instead of using many variables.
(outline--make-button-overlay): Use value 'insert' of
'outline-minor-mode-use-buttons'.
(outline--insert-open-button, outline--insert-close-button)
(outline--fix-up-all-buttons, outline--fix-buttons-after-change):
Handle values of outline-minor-mode-use-buttons instead of using
many variables.