New command 'eww-copy-alternate-url'

New command 'eww-copy-alternate-url'

This adds a new command to EWW that copies an alternate link to the
currently visited page into the kill ring.  This is useful for
subscribing to website feeds, etc.

* lisp/net/eww.el (eww--alternate-urls, eww-read-alternate-url):
 New functions.
(eww-copy-alternate-url): New command.
(eww-mode-map): Bind it to 'A'.

* doc/misc/eww.texi (Basics): Document it.

* etc/NEWS: Announce it.
(Bug#64126)