Generate NEWS.XX.Y.html file for Emacs website

[Stefan Kangas <stefankangas@gmail.com>]

[I'm creating a new thread for generating HTML versions of etc/NEWS for
 the website.  This is continued from the discussion starting here:
 https://lists.gnu.org/r/emacs-devel/2022-09/msg00759.html ]

Eli Zaretskii <eliz@gnu.org> writes:

> [P]lease describe what is needed (which commands and packages, and how
> to use them) to produce this HTML version.

The command is:

    ./src/emacs -l admin/admin.el -f make-news-html-file
    mv etc/NEWS.XX.Y.html $EMACS_WEBROOT/news
    cd $EMACS_WEBROOT
    cvs commit -m "Update NEWS.XX.Y.html" news/NEWS.XX.Y.html

Please see `make-news-html-file' in admin/admin.el for more details.
It's still a work in progress, of course, but I'm happy to work on it.

> Also, how will this HTML version be maintained, after it is produced.

The easiest option is to just leave the old generated HTML files alone
and update them only if absolutely necessary.

But it'd be better if we could generate HTML versions of all etc/NEWS.*
files whenever we make a new release.  I didn't yet look into this, so I
don't know how realistic this is, but I have a hunch that it's doable.
Maybe we will only be able to do it for releases newer than Emacs 21 (or
something) though.

We would then also need to add some code to delete all the Emacs 28.2
stuff from "NEWS.28.1.html".  Maybe we would want to trim out the Emacs
28.1 headlines from "NEWS.28.2.html"?  I'm not sure.

The second option has the added benefit that any fixes or additions we
make in old NEWS files would eventually propagate to the website.