bug#46236: 26.1; explicit the info files installation

[Gregory Heytings]

> > A suggestion: when the info files bundled with Emacs cannot be found, 
> > download them from info.gnu.org (after asking the user if they're okay 
> > with that) and install them into ~/.emacs.d/info.
>
> We already have the latest manuals at
>
>  https://www.gnu.org/software/emacs/manual/emacs.html
>
> but what you suggest will require to maintain several versions back, 
> which is a non-trivial complication of the release procedures that are 
> already quite complex and tedious.  So I don't think we should take this 
> issue to be our problem to that level.

In fact this makes it even easier than what I thought: it would suffice to 
add 60 files in the https://www.gnu.org/software/emacs/manual/info/ 
directory.  Of course there's the version problem.  Perhaps an 
"emacs-version" file could be added in that directory, it would be 
compared against emacs-version, with a warning when the files are for a 
more recent version.