Feature: Convert Markdown to EPUB

[fantasyczl]

This PR adds support for converting Markdown files into EPUB format, making it easier to read notes in apps like Apple Books and WeChat Read.

Highlights:

Added EPUB export functionality
Compatible with multiple ebook platforms
Looking forward to your review!

[yingang]

Encountered some tex related warnings when trying to build epub with your scripts and not sure if it was caused by my own internet connection.

Anyway, thanks for your contribution!

[fantasyczl]

I don't see any TeX warnings on my computer. Could you provide some warning messages so I can take a look at what the issue is?

[yingang]

I don't see any TeX warnings on my computer. Could you provide some warning messages so I can take a look at what the issue is?

除了少数几个这种：

[WARNING] Note with key '译注i' defined at /home/yingang/ddia/ch3.md line 173 column 1 but not used.
[WARNING] Note with key 'viii' defined at /home/yingang/ddia/ch9.md_chunk line 3 column 1 but not used.

其它基本都是这样的，看上去像是连接的问题：

[WARNING] Could not fetch resource https://latex.codecogs.com/png.latex?t_x: PandocHttpError "https://latex.codecogs.com/png.latex?t_x" (HttpExceptionRequest Request {
    host                 = "latex.codecogs.com"
    port                 = 443
    secure               = True
    requestHeaders       = []
    path                 = "/png.latex"
    queryString          = "?t_x"
    method               = "GET"
    proxy                = Nothing
    rawBody              = False
    redirectCount        = 10
    responseTimeout      = ResponseTimeoutDefault
    requestVersion       = HTTP/1.1
    proxySecureMode      = ProxySecureWithConnect
  }
   ConnectionTimeout)

[fantasyczl]

1. 前两个问题，我也碰到过，以前没有太在意，仔细看了一下，在 ch03.md 和 ch09.md 中有两个引用不对。已经修正了，在fix: 修复错误的引用 #353
2. 后面的问题看起来是网络，可以尝试使用浏览器打开： https://latex.codecogs.com/png.latex?t_x 网络正常应该能看到一个图标。可以试试看看。

[yingang]

谢谢，前一个已经合并了，后一个警告里的链接试了下倒是可以直接打开，但运行脚本的时候还是会有这些警告，不知道是为什么了，从字面上看还是网络相关。