Add benefits for partners to partners/become.mdwn

similar to https://www.fsf.org/patrons/benefits

What do Donors/Sponsors/Patrons receive when they donate?
Let’s keep in mind that we could give them some T-Shirts.
And explicit that they can advertise that they help Tails.

Related issues

• Related to #6972 (closed)

Original created by @u on 12514 (Redmine)