docview: imenu access to table of contents

docview: imenu access to table of contents

* lisp/doc-view.el (doc-view-imenu-enabled): user option to disable
imenu generation.
* lisp/doc-view.el (doc-view--outline-rx):
(doc-view--pdf-outline, doc-view--imenu-subtree, doc-view-imenu-index):
functions implementing the imenu index generation via mutool.
* lisp/doc-view.el (doc-view-imenu-setup, doc-view-mode): setup of the
new functionality in doc-view mode.
* lisp/imenu.el (imenu-submenus-on-top):
(imenu--split-menu): new local variable to optionally inhibit
grouping of entries with children at the top of imenu menus.
* doc/emacs/misc.texi: documentation for the new functionality
(bug#58103).