doc: replace a couple of broken gmane links

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>