doc: stop advertising mailing list subscription

It's still available, but no point in advertising something
which increases the dependency on a centralized subscriber list.

Subscription will never be required to write to our
centralization-resistant public inbox (which anybody can
read/mirror using HTTP(S) or NNTP).

The mailing list has only attracted a tiny handful of
subscribers since Rubyforge died, and more than half of the
subscribers imported from Rubyforge have bounced off
(or unsubscribed).

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