feat: enhance blog post and page schema with new properties

- Added optional `basedOn` field to blog post schema for better content attribution.
- Updated blog post markdown to include `basedOn` reference for improved context.
- Refactored page schema to replace deprecated website schema, enhancing structured data representation.
- Adjusted 404 and index pages to utilize the new page schema for consistency and SEO improvements.

src/pages/404.astro

@@ -1,15 +1,17 @@
 ---
 import Layout from "../layouts/BaseLayout.astro";
-import websiteSchema from "../utils/schemas/websiteSchema";
+import pageSchema from "../utils/schemas/pageSchema";
 
 const title = "404 — Page Not Found | Valentin Popov";
 const description = "The page you're looking for doesn't exist!";
 const lang = "en";
 
-const schema = websiteSchema({
+const schema = pageSchema({
 	siteUrl: new URL("/", Astro.site).toString(),
+	page: "/404",
 	title,
 	description,
+	lang,
 });
 ---