refactor: replace postSlug with Astro content slug

* refactor: replace postSlug with Astro content slug

Remove postSlug from blog schema. Refactor articles that use postSlug. Remove slugify function from slugify util file. Replace slugify logic with Astro's content collections slug logic.

Closes: #186

* docs: update docs for removed slugify contents

* refactor: remove unused import

src/utils/slugify.ts

@@ -1,11 +1,5 @@
 import { slug as slugger } from "github-slugger";
-import type { CollectionEntry } from "astro:content";
 
 export const slugifyStr = (str: string) => slugger(str);
 
-const slugify = (post: CollectionEntry<"blog">["data"]) =>
-  post.postSlug ? slugger(post.postSlug) : slugger(post.title);
-
 export const slugifyAll = (arr: string[]) => arr.map(str => slugifyStr(str));
-
-export default slugify;