Add commands xref-next-group and xref-prev-group

Add commands xref-next-group and xref-prev-group

* lisp/progmodes/xref.el (xref-next-group): New command that navigates
to the first item of the next xref group (typically a file).
(xref-prev-group): New command that navigates
to the first item of the next xref group (typically a file).
(xref--xref-buffer-mode-map): Bound the new commands to 'N' and 'P',
respectively.
* doc/emacs/maintaining.texi (Xref Commands): Document the new
commands in the Emacs manual.
* etc/NEWS: Announce them (bug#45089).