Add imenu index function for DjVu files in doc-view

Add imenu index function for DjVu files in doc-view

* lisp/doc-view.el (doc-view-pdfdraw-program): Prefer mutool
over other names.
(doc-view-imenu-enabled): Tweak the default value to check for
'djvused', and make it obsolete.
(doc-view--djvu-outline, doc-view--parse-djvu-outline): Add new
functions to return imenu index for a Djvu file.
(doc-view--outline): Add new function to create the imenu index
depending on the file type.
(doc-view--outline): Document new possible variable value.
(doc-view-imenu-index): Use the above function instead.
(doc-view-imenu-setup): Try to create the imenu index
unconditionally.
* doc/emacs/misc.texi (DocView Navigation): Mention index
creation using 'djvused' too.
* etc/NEWS: Announce the change.  (Bug#73530)

(cherry picked from commit d6dd71cd56e73aaf998d27225249edf661ead488)