docs: add giscus comments system (#96)

* chore: rename comment to comments

* docs: add giscus comments

* docs: update

exampleSite/content/docs/advanced/comments.md

@@ -0,0 +1,39 @@
+---
+title: Comments System
+linkTitle: Comments
+---
+
+Hextra supports adding comments system to your site.
+Currently [giscus](https://giscus.app/) is supported.
+
+<!--more-->
+
+## giscus
+
+[giscus](https://giscus.app/) is a comments system powered by [GitHub Discussions](https://docs.github.com/en/discussions). It is free and open source.
+
+To enable giscus, you need to add the following to the site configuration file:
+
+```yaml {filename="hugo.yaml"}
+params:
+  comments:
+    enable: false
+    type: giscus
+
+    giscus:
+      repo: <repository>
+      repoId: <repository ID>
+      category: <category>
+      categoryId: <category ID>
+```
+
+The giscus configurations can be constructed from the [giscus.app](https://giscus.app/) website. More details can also be found there.
+
+Comments can be enabled or disabled for a specific page in the page front matter:
+
+```yaml {filename="content/docs/about.md"}
+---
+title: About
+comments: true
+---
+```