Improve error message in DAST CI template

When DAST_WEBSITE is not defined in a pipeline, the template will still
attempt to run the analyzer but fail without any explanation.

This change in the template will output an error and a link to the
documentation if the variable is not set.

changelogs/unreleased/sh-improve-dast-template-error.yml

+---
+title: Improve error message in DAST CI template
+merge_request: 29388
+author:
+type: other

lib/gitlab/ci/templates/Security/DAST.gitlab-ci.yml

@@ -22,6 +22,7 @@ dast:
   allow_failure: true
   script:
     - export DAST_WEBSITE=${DAST_WEBSITE:-$(cat environment_url.txt)}
+    - if [ -z "$DAST_WEBSITE$DAST_API_SPECIFICATION" ]; then echo "Either DAST_WEBSITE or DAST_API_SPECIFICATION must be set. See https://docs.gitlab.com/ee/user/application_security/dast/#configuration for more details." && exit 1; fi
     - /analyze
   artifacts:
     reports: