Document the tails-press mailing list

The “talk to us” section of https://tails.boum.org/contribute/how/promote/ should be removed, and a link to the list provided instead.

https://tails.boum.org/press/ should also link to the list.

Feature Branch: frithjof/tails-press

Original created by @BitingBird on 7574 (Redmine)

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