feat: add number of posts config for home page (#281)

* Add a SITE config parameter for posts in index page

* docs: update blog post for `postPerIndex` config

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Co-authored-by: satnaing <satnaingdev@gmail.com>

src/config.ts

@@ -8,6 +8,7 @@ export const SITE: Site = {
   title: "AstroPaper",
   ogImage: "astropaper-og.jpg",
   lightAndDarkMode: true,
+  postPerIndex: 4,
   postPerPage: 3,
   scheduledPostMargin: 15 * 60 * 1000, // 15 minutes
 };