chore: add validation for PR description and checklist

.github/scripts/validate_pr.py

@@ -38,7 +38,7 @@
 
 # Check if the first two checklist items are selected
 # This pattern allows for optional spaces and both lowercase/uppercase x between brackets
-contrib_checked = re.search(r"- \[\s*[xX]\s*\] I have read the \[`CONTRIBUTING`\] document", pr_body)
+contrib_checked = re.search(r"- \[\s*[xX]\s*\] I have read the \[`CONTRIBUTING`\]\(https://github.com/nginx/agent/blob/main/docs/CONTRIBUTING\.md\)", pr_body)
 install_checked = re.search(r"- \[\s*[xX]\s*\] I have run `make install-tools`", pr_body)
 
 if not contrib_checked: