remove MATHJAX env var and replace with KaTeX

packages/app/config/ci/.env.local.for-ci

@@ -1,2 +1 @@
 FORMAT_NODE_LOG=true
-MATHJAX=1

packages/app/resource/locales/en_US/sandbox-math.md

@@ -1,6 +1,6 @@
 # :pencil: Math
 
-See [MathJax](https://www.mathjax.org/).
+See [KaTeX](https://katex.org/).
 
 ## Inline Formula
 

packages/app/resource/locales/ja_JP/sandbox-math.md

@@ -1,6 +1,6 @@
 # :pencil: Math
 
-See [MathJax](https://www.mathjax.org/).
+See [KaTeX](https://katex.org/).
 
 ## Inline Formula
 

packages/app/resource/locales/zh_CN/sandbox-math.md

@@ -1,6 +1,6 @@
 # :pencil: Math
 
-See [MathJax](https://www.mathjax.org/).
+See [KaTeX](https://katex.org/).
 
 ## Inline Formula