Add KeyStatic CMS integration

Features added:
- KeyStatic Core and Astro integration installed
- Markdoc integration for content rendering
- keystatic.config.ts with blog post schema
- Local storage mode for content management
- Conditional loading via SKIP_KEYSTATIC env var
- Netlify build command updated to skip KeyStatic in production

KeyStatic Admin UI available at:
- Local: http://localhost:4321/keystatic
- Production: Disabled for security

Co-Authored-By: Claude <noreply@anthropic.com>

astro.config.ts

@@ -17,6 +17,9 @@ import { readingTimeRemarkPlugin, responsiveTablesRehypePlugin, lazyImagesRehype
 
 import react from '@astrojs/react';
 
+import markdoc from '@astrojs/markdoc';
+import keystatic from '@keystatic/astro';
+
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 const hasExternalScripts = true;
@@ -60,7 +63,7 @@ export default defineConfig({
     Logger: 1,
   }), astrowind({
     config: './src/config.yaml',
-  }), react()],
+  }), react(), markdoc(), ...(process.env.SKIP_KEYSTATIC ? [] : [keystatic()])],
 
   i18n: {
     defaultLocale: 'tr',