Add new Tails UX mailing-list information to the documentation

A mailinglist for tails-ux@boum.org has been created.

The scope and information for that ML should be added to the documentation (contribute/design?)

Feature Branch: 451f doc/uxml

Original created by @u on 7535 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information