Add The Onsite to interview resources.

Include a timed, interactive system design practice option with AI feedback in the interview resources section.

Author: marcuslam-coder

readme.md

@@ -56,6 +56,8 @@ Started By Gabriel Leon de Mattos
 
 - [Practice system design problems using AI on Codemia.io](https://codemia.io) - A tool which allows you to practice system design problems interactively like an interview with AI. There's iterative feedback and final evaluation which scores your performance
 
+- [The Onsite](https://theonsite.dev) - [Paid 💵] - Timed system design interview practice with an interactive diagramming canvas, AI feedback, and reference answers.
+
 ## Advanced
 
 - [Distributed Computing](https://en.wikipedia.org/wiki/Distributed_computing) - Wikipedia article broadening the view of distributed system design.