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/layouts/Posts.astro

@@ -6,7 +6,6 @@ import Header from "@components/Header.astro";
 import Footer from "@components/Footer.astro";
 import Card from "@components/Card";
 import LinkButton from "@components/LinkButton.astro";
-import slugify from "@utils/slugify";
 import type { CollectionEntry } from "astro:content";
 
 export interface Props {
@@ -26,8 +25,8 @@ const next = pageNum < totalPages ? "" : "disabled";
   <Main pageTitle="Posts" pageDesc="All the articles I've posted.">
     <ul>
       {
-        posts.map(({ data }) => (
-          <Card href={`/posts/${slugify(data)}`} frontmatter={data} />
+        posts.map(({ data, slug }) => (
+          <Card href={`/posts/${slug}`} frontmatter={data} />
         ))
       }
     </ul>