I've been locally testing configuration for Company documentation (to
add it to GNU ELPA) and noticed that an HTML version of the manual is
automatically generated from the '.texi' file. The generated HTML file
is linked under the label 'Manual' on the package page and served from
the 'archive-devel/doc/company/' directory.

I was pleased to see that automatic generation but there were two issues
(namely, images and styles).

The user manual for Company is shipped with the images; and it is
currently impossible to configure the addition of them to the
'elpaa--doc-subdirectory'.

My initial idea for fixing this issue was to add a property
':doc-asset', which could be configured to '(FROM . TO)' directories
names, then used for moving images to the 'archive-devel/doc/company/'
folder, as shown below:

#+BEGIN_SRC diff
diff --git a/elpa-admin.el b/elpa-admin.el
index d94cc7fd73..23bb582ceb 100644
--- a/elpa-admin.el
+++ b/elpa-admin.el
@@ -1999,7 +1999,11 @@ directory; one of archive, archive-devel."
 	    (expand-file-name elpaa--doc-subdirectory tarball-dir)))))
     (when html-dir
       (when (not (file-readable-p html-dir)) ;FIXME: Why bother testing?
-	(make-directory html-dir t)))
+	    (make-directory html-dir t))
+      (when-let ((assets (elpaa--spec-get pkg-spec :doc-assets)))
+        (cl-loop for (from . to) in assets
+                 do (rename-file (expand-file-name from dir)
+                                 (expand-file-name to html-dir) t))))

     (plist-put (cdr pkg-spec) :internal--html-docs nil)
     (dolist (f docfiles)
-- 
2.34.1
#+END_SRC

That worked fine during my tests but thinking about how to apply CSS
styles to the manual (including to control the size of the shown
images), I've got the second idea: Company already has an online version
of the user manual with all the proper styles and images in place, so
why not link directly to it?

So, the suggested patch provides a way to do that. It introduces and
uses two new properties: ':doc-html' and ':resources'.

With it, for example, Company setup could be configured this way:

#+BEGIN_SRC lisp
("company"
  :url "https://github.com/company-mode/company-mode.git"
  :shell-command "mkdir images; mv doc/images/small images/"
  :doc "doc/company.texi"
  :doc-html ignore
  :resources (("User Manual" . "https://company-mode.github.io/manual/index.html"))
  :auto-sync t)
#+END_SRC


I also attach a version with a bit different (IMO less robust/clear)
approach. It introduces/uses one new property instead: ':doc-links'.