feat: migrate learning-hub articles into Astro website

- Add Astro Content Collection for learning-hub articles
- Move 5 fundamentals articles into website/src/content/learning-hub/
- Create ArticleLayout.astro for rendering markdown articles
- Create index page listing all articles in recommended reading order
- Create dynamic [slug].astro route for individual articles
- Add Learning Hub to main navigation and homepage cards
- Add article prose and index page CSS styles
- Update internal links to use website URLs

website/src/content.config.ts

@@ -0,0 +1,20 @@
+import { defineCollection, z } from "astro:content";
+import { glob } from "astro/loaders";
+
+const learningHub = defineCollection({
+  loader: glob({ pattern: "**/*.md", base: "./src/content/learning-hub" }),
+  schema: z.object({
+    title: z.string(),
+    description: z.string(),
+    authors: z.array(z.string()).optional(),
+    lastUpdated: z.string().optional(),
+    estimatedReadingTime: z.string().optional(),
+    tags: z.array(z.string()).optional(),
+    relatedArticles: z.array(z.string()).optional(),
+    prerequisites: z.array(z.string()).optional(),
+  }),
+});
+
+export const collections = {
+  "learning-hub": learningHub,
+};