希望增加markdown文件中对相对路径图片的展示功能

### Please make sure of the following things

- [X] I have read the [documentation](https://alist.nn.ci).
- [X] I'm sure there are no duplicate issues or discussions.
- [X] I'm sure this feature is not implemented.
- [X] I'm sure it's a reasonable and popular requirement.

### Description of the feature / 需求描述

markdown中有时候会在文档中放相对路径下的图片，例"./docs/test.jpg"，alist似乎无法读取到这个图片，旧版可以把大佬编写的一段js代码放在自定义头部内实现这个功能，但新版不行，希望能增加这个功能，感谢