contents(sd): add System Design Newsletter (#618)

Co-authored-by: Neo Kim <hello@systemdesign.one>

apps/website/contents/system-design.md

@@ -48,6 +48,7 @@ System design content is still work-in-progress, but the following are some reso
 ## Quality resources
 
 - ["Front End System Design Guidebook" by GreatFrontEnd](https://www.greatfrontend.com/system-design) - A free system design guidebook by yours truly, focused on system design for the front end, e.g. how to design Facebook's News Feed, Autocomplete components, Image Carousels and more.
+- [System Design Newsletter](https://newsletter.systemdesign.one/) - Weekly newsletter on system design. Also get the powerful system design template for FREE.
 - [ByteByteGo](https://bytebytego.com?fpr=techinterviewhandbook) - This is a new System Design course by Alex Xu, author of the System Design Interview books, a bestseller on Amazon. The course covers system designs basics, then goes into deep dives of the design of over 10 famous common products (e.g. [Designing YouTube](https://bytebytego.com/courses/system-design-interview/design-youtube?fpr=techinterviewhandbook), Facebook Newsfeed, etc) and multiple big data and storage systems (e.g. [Designing a Chat System](https://bytebytego.com/courses/system-design-interview/design-a-chat-system?fpr=techinterviewhandbook)). For each deep dive, concepts are explained and comprehensive diagrams are used, making it very approachable regardless of seniority level.
 - ["Grokking the System Design Interview" by Design Gurus](https://designgurus.org/link/kJSIoU?url=https%3A%2F%2Fdesigngurus.org%2Fcourse%3Fcourseid%3Dgrokking-the-system-design-interview) - This is probably the most famous system design interview course on the internet and what makes it different from most other courses out there is that it is purely text-based, which is great for people who prefer reading over watching videos (such as myself!). It contains a repository of the popular system design problems along with a glossary of system design basics. I've personally completed this course and highly recommended many others to use this.
 - ["Grokking the Advanced System Design Interview" by Design Gurus](https://designgurus.org/link/kJSIoU?url=https%3A%2F%2Fdesigngurus.org%2Fcourse%3Fcourseid%3Dgrokking-the-advanced-system-design-interview) - I haven't tried this but it's by the same people who created "Grokking the System Design Interview", so it should be good! In my opinion you probably wouldn't need this unless you're very senior or going for a specialist position.