Resolve "Update DAST docs for specifying target URL via file"

What does this MR do?

Updates documentation to include the ability to specify the DAST target URL via an environment_url.txt file (functionality added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26957)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #10914 (closed) and #10851 (closed)