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/pages/rss.xml.ts

@@ -1,7 +1,6 @@
 import rss from "@astrojs/rss";
 import { getCollection } from "astro:content";
 import getSortedPosts from "@utils/getSortedPosts";
-import slugify from "@utils/slugify";
 import { SITE } from "@config";
 
 export async function GET() {
@@ -11,8 +10,8 @@ export async function GET() {
     title: SITE.title,
     description: SITE.desc,
     site: SITE.website,
-    items: sortedPosts.map(({ data }) => ({
-      link: `posts/${slugify(data)}`,
+    items: sortedPosts.map(({ data, slug }) => ({
+      link: `posts/${slug}`,
       title: data.title,
       description: data.description,
       pubDate: new Date(data.pubDatetime),