bug#58196: Trivial update to ediprolog

[Stefan Monnier]

> For example, we could put factorial.png in
> /images/<package-name>/factorial.png.  The only cost is a bit of

But the page which links to it is not inside `/images/<package-name>/`
so the relative link will still fail.

I think we need to first move the packages's webpages from

   https://elpa.gnu.org/packages/<PKG>.html

to

   https://elpa.gnu.org/packages/<PKG>/index.html

and then we can more easily copy image files and anything else to
that subdirectory.

We have similar needs for the HTML-rendered `:doc`s which are placed into
https://elpa.gnu.org/packages/doc/<MANUAL>.html so we'd also want to
copy somefiles to that `doc` subdir.  The problem is a bit worse there,
because that subdir is shared between all the packages so as to make it
possible to have relative links between manuals of different packages.
IOW we probably need to enforce that all the files that need to be
copied into the `doc` subdir are prefixed by the package name.


        Stefan