docs: add Abhijeet's math-wiki to the showcase (#228)

content/notes/showcase.md

@@ -18,5 +18,6 @@ Want to see what Quartz can do? Here are some cool community gardens :)
 - [sspaeti.com's Second Brain](https://brain.sspaeti.com/)
 - [oldwinterの数字花园](https://garden.oldwinter.top/)
 - [SethMB Work](https://sethmb.xyz/)
+- [Abhijeet's Math Wiki](https://abhmul.github.io/quartz/Math-Wiki/)
 
 If you want to see your own on here, submit a [Pull Request adding yourself to this file](https://github.com/jackyzha0/quartz/blob/hugo/content/notes/showcase.md)!