doc: replace a couple of broken gmane links
Replace a couple of broken links to gmane with links to other
archives. See commit
54471fdcc3 ("README: replace gmane link with
public-inbox", 2016-12-15) for prior art.
With this change there's still 4 references left in the code:
$ git grep -E '(article|thread)\.gmane.org' -- |grep -v RelNotes|wc -l
4
I couldn't find alternative links for those.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>