Add new Gnus summary commands and keystrokes for "unseen" navigation

* doc/misc/gnus.texi (Choosing Commands): Document them.
* lisp/gnus/gnus-sum.el (gnus-summary-next-unseen-article)
(gnus-summary-prev-unseen-article): New commands and
keystrokes (bug#35213).
1 job for master in 56 minutes and 57 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #3524
test-all

00:56:57