Add dynamic generation of sitemap and robots.txt

[halilatilla]

I've noticed that the project was missing a sitemap and a robots.txt file. Including these can significantly improve the site's SEO and accessibility for search engines.

Here are the key changes implemented:

• Included next-sitemap package in devDependencies.
• Introduced next-sitemap.config.js for managing dynamic file generation.
• Adjusted postbuild script in package.json to utilize next-sitemap.
• Set the output directory for generated files to public.
• Listed these dynamically created files in .gitignore to prevent committing.

Upon completion of the build process, these files will be available within the public folder.

public folder

robots.txt

sitemap.xml

sitemap-0.xml