docs: adds March Update link to newsletter archive (#3643)

Author: indexing

docs/reference/newsletter-archive.md

@@ -9,8 +9,11 @@ If you’re interested in receiving this update delivered directly to your email
 - [The January Update](https://5519226.hs-sites.com/cal-itp-benefits-the-january-update-2026)
 
 - [The February Update](https://5519226.hs-sites.com/cal-itp-benefits-the-february-update-2026)
+
+- [The March Update](https://5519226.hs-sites.com/cal-itp-benefits-the-march-update-2026)
+
 <!--
-- [The March Update]()
+- [The April Update]()
   -->
 
 ### 2025