Add MathJax rendering to the markdown viewer

**is your feature request related to a problem? Please describe.**
I use copyparty as a way to render my handwritten notes so that I can search them. Alot of my notes involve writing math and using [Mathjax](https://www.mathjax.org/) to format it all

**Describe the idea / solution you'd like**
A plugin to add mathjax rendering

This also may be useful to those who are using copyparty as a way to host the content of an obsidian vault

查看 copyparty 前端代码中的 Markdown 查看器，可能位于 JavaScript/HTML 模板中。找到 Markdown 的渲染位置，并添加 MathJax 脚本的引入和初始化。上传一个包含 LaTeX 数学表达式的 Markdown 文件进行测试，并确认它们能在浏览器中正确渲染。

由索引模型根据 Issue 内容生成。