Quarterly reminder to tails-project@ about sponsorship (from [misc])

Originally created by @intrigeri on #14725 (Redmine)

We want to let all active contributors know that it’s an option to be reimbursed when attending events on behalf of Tails. Once we have publicly available doc about how it works, we’ll set up an automated reminder that points tails-project@ to it.

Parent Task: tails/private#12632

Related issues

• Related to #7763 (closed)
• Blocked by tails#14727 (closed)