refactor: flatten URL structure

- Remove /docs/ prefix and category subfolders from URLs
- URLs are now flat: /introduction, /commands, etc.
- Index redirects to /introduction
- Rename overview files to fundamentals-overview and advanced-overview

Author: CalvinAllen

astro.config.mjs

@@ -19,8 +19,5 @@ export default defineConfig({
       langs: ['csharp', 'xml', 'json', 'powershell', 'bash'],
       wrap: true
     }
-  },
-  redirects: {
-    '/docs': '/docs/getting-started/introduction'
   }
 });