feat: add new features including SEO, sitemap, and subjects management

- Create robots.txt to allow search engine indexing and specify sitemap location.
- Implement ajouterSujet.js for adding new subjects to sujets.json.
- Develop dynamic subject pages in page.tsx using static generation with metadata.
- Add contact, error, not found, and privacy policy pages for improved user experience.
- Generate sitemap.xml for better search engine visibility.
- Create SEO component for managing page metadata.
- Populate sujets.json with detailed subject information for dynamic content rendering.

scripts/ajouterSujet.js

@@ -0,0 +1,24 @@
+import fs from 'fs';
+
+/**
+ * Ajoute un nouveau sujet au fichier sujets.json
+ * @param {object} sujet - Objet sujet à ajouter
+ */
+export function ajouterSujet(sujet) {
+  const filePath = './src/data/sujets.json';
+  const data = JSON.parse(fs.readFileSync(filePath, 'utf-8'));
+  data.push(sujet);
+  fs.writeFileSync(filePath, JSON.stringify(data, null, 2));
+}
+
+/**
+ * Exemple d’utilisation :
+ * ajouterSujet({
+ *   slug: 'nouveau-sujet',
+ *   title: 'Titre',
+ *   description: 'Description',
+ *   image1: { src: '/images/xxx.jpg', alt: '...' },
+ *   image2: { src: '/images/yyy.jpg', alt: '...' },
+ *   sections: [ ... ]
+ * });
+ */