Redesign and improve the help-for-help (C-h C-h) command

Redesign and improve the help-for-help (C-h C-h) command

* lisp/help.el (help-for-help): Redesign help screen; add sections,
rearrange and reword.
(help-for-help-header): New face.
(help--for-help-make-commands, help--for-help-make-sections): New
functions.
(help-for-help-buffer-name): New variable.

* lisp/help-macro.el (make-help-screen): New optional argument
BUFFER-NAME.  Fontify keys.

This change was discussed in:
https://lists.gnu.org/r/emacs-devel/2021-02/msg01695.html
https://lists.gnu.org/r/emacs-devel/2021-03/msg00670.html
https://lists.gnu.org/r/emacs-devel/2021-04/msg00292.html