doc: replace public-inbox links with lore.kernel.org

doc: replace public-inbox links with lore.

Since we're now recommending lore.kernel.org (and because the
public-inbox.org domain might eventually go away), let's update our
internal references to use it, too. That future-proofs our references,
and sets the example we want people to follow.

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