fix: Create robots.txt template to ensure proper sitemap reference

- Removed static robots.txt that wasn't being deployed properly
- Created robots.txt template in theme layouts
- Uses Hugo template to generate proper sitemap URL
- Should fix incomplete robots.txt deployment

Author: Debdyut

portfolio/static/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: {{ .Site.BaseURL }}sitemap.xml