fix: markdown and katex rendering problem

[zayn7lie]

According to https://discourse.gohugo.io/t/braces-in-latex-get-escaped/39742/5; https://discourse.gohugo.io/t/use-goldmark-mathjax-extension/25721/8; and https://discourse.gohugo.io/t/mathjax-rendering-problems-due-to-backslashes-interpretation-by-markdown-processor/30482, there are problems about rendering order about Markdown and KaTeX with Hugo Goldmark. So I add a partial which could fix the problem in the process of rendering.

[CaiJimmy]

Hi, thanks for the pull request. This issue has already been fixed by Hugo. See #1019 and https://gohugo.io/content-management/mathematics/