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/types.ts

@@ -8,6 +8,7 @@ export type Site = {
   title: string;
   ogImage?: string;
   lightAndDarkMode: boolean;
+  postPerIndex: number;
   postPerPage: number;
   scheduledPostMargin: number;
 };