feat: add [giscus](https://giscus.app) comment

[mivinci]

You can't just add the giscus template or its entire configuration into Minima for the reasons below:

1. When switching to the dark/light mode, giscus needs to be notified and changes as well.
2. Users, as lazy as they are, need us to make some simple and usable presets of any plugin's configuration.

So I'm closing this PR.

[qaqland]

So... Is there a simpler way to add <script> or <style> into my page without editing the minima theme? Maybe I can do the configuration edit straightly in template html file. 祝好

[mivinci]

应该是没有的，你要加 giscus 吗，我打算加但还没时间弄。

[qaqland]

我去看看文档，想想怎么弄简单一点