Add challenge day signup email

SR2026/2026-01-06-challenge-days.md

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+---
+to: SR2026 teams
+subject: SR2026 Challenge Days
+---
+
+Hi,
+
+[Challenge Days][tech-days] (formally known as Tech Days) are opportunities for teams to spend a whole day working on completing challenges with lots of help available. -- from a mix of long-experienced volunteers and more recent ex-competitors. They're also an opportunity to see how other teams are doing, get more direct help with your robots, discuss your strategy.
+
+We provide a space for you to work in, with power and internet access, as well as volunteers able to help you with your kits and hands-on guidance with your robots.
+
+We're planning on running 2 challenge days:
+
+- [**24th January 2026** - at Raspberry Pi Foundation in Cambridge](https://studentrobotics.org/events/sr2026/cambridge-challenge-day-january)
+- [**21st February 2026** - at Red River Software in Horsham](https://studentrobotics.org/events/sr2026/horsham-challenge-day-february)
+
+If you're interested in attending any of these events, please complete [this form][tech-day-signup]. Volunteers may also be around virtually on these days for support, should you not be able to attend any events in person.
+
+Spaces are limited at each event, so be sure to let us know soon. We will be allocating and confirming places for it as we receive applications, so sign up soon to avoid disappointment.
+
+-- Student Robotics
+
+[tech-days]: https://studentrobotics.org/docs/robots_101/tech_days
+[tech-day-signup]: https://forms.gle/ydaeuV4ckvZMWL7Z8