bug#44504: 27.1; feature request: doc-view support for Djvu documents

[Roland Winkler]

On Mon Nov 9 2020 Roland Winkler wrote:
> Thank you!  My apologies for being picky: How about also updating
> the first line of doc-view.el to
> 
> ;;; doc-view.el --- Document viewer for Emacs -*- lexical-binding: t -*-

...And the section "Requirements" is likewise outdated. How about
adding:

;; Djvu documents require `ddjvu' (from DjVuLibre).
;; ODF files require `soffice' (from LibreOffice).

Actually, I do not know about `soffice'.  Is this also part of
some (outdated?) Openoffice?