Add GitHub Pages deployment and Decap CMS support

- Add GitHub Actions workflow for automatic deployment to GitHub Pages
- Configure Decap CMS for Turkish blog post management
- Add Netlify Identity integration for authentication
- Create admin redirect page and routing
- Add comprehensive setup documentation
- Configure proper Turkish field labels and defaults
- Enable image uploads and media management

Site will now auto-deploy on every push to main branch.
Access CMS at: https://ym1ktc.github.io/ARC-Web-Sitesi/admin/

🤖 Generated with Claude Code

public/decapcms/config.yml

@@ -5,25 +5,21 @@ backend:
 media_folder: 'src/assets/images'
 public_folder: '/_astro'
 
+site_url: https://ym1ktc.github.io/ARC-Web-Sitesi
+display_url: https://ym1ktc.github.io/ARC-Web-Sitesi
+
 collections:
   - name: 'post'
-    label: 'Post'
-    folder: 'src/content/post'
+    label: 'Blog Yazıları'
+    folder: 'src/data/post'
     create: true
+    slug: '{{year}}-{{month}}-{{day}}-{{slug}}'
     fields:
-      - { label: 'Title', name: 'title', widget: 'string' }
-      - { label: 'Excerpt', name: 'excerpt', widget: 'string' }
-      - { label: 'Category', name: 'category', widget: 'string' }
-      - {
-          label: 'Tags',
-          name: 'tags',
-          widget: 'list',
-          allow_add: true,
-          allow_delete: true,
-          collapsed: false,
-          field: { label: 'Tag', name: 'tag', widget: 'string' },
-        }
-      - { label: 'Image', name: 'image', widget: 'string' }
-      - { label: 'Publish Date', name: 'publishDate', widget: 'datetime', required: false }
-      - { label: 'Author', name: 'author', widget: 'string' }
-      - { label: 'Content', name: 'body', widget: 'markdown' }
+      - { label: 'Başlık', name: 'title', widget: 'string' }
+      - { label: 'Yayın Tarihi', name: 'date', widget: 'datetime', format: 'YYYY-MM-DD HH:mm:ss' }
+      - { label: 'Yazar', name: 'author', widget: 'string', default: 'TA1SPH' }
+      - { label: 'Kategoriler', name: 'categories', widget: 'list', allow_add: true, allow_delete: true, collapsed: false, field: { label: 'Kategori', name: 'category', widget: 'string' } }
+      - { label: 'Etiketler', name: 'tags', widget: 'list', allow_add: true, allow_delete: true, collapsed: false, field: { label: 'Etiket', name: 'tag', widget: 'string' } }
+      - { label: 'Düzen', name: 'layout', widget: 'string', default: 'post' }
+      - { label: 'Öne Çıkan Görsel', name: 'image', widget: 'image', media_folder: '/src/assets/images', public_folder: '~/assets/images', required: false }
+      - { label: 'İçerik', name: 'body', widget: 'markdown' }