Docs: Update DAST documentation to clarify required variables for CI/CD job setup

What does this MR do and why?

This MR is part of improving DAST documentation for users unfamiliar with how to set DAST up, this MR update is "Part 2" of the linked issue. We need to update our docs on how to Create a DAST CI/CD job to make it clear that the user needs to set valid DAST variables for example DAST_TARGET_URL in order for the DAST job to succeed. Simply including the DAST template and referencing it without putting in and DAST variables will result in the job failing because the user needs to configure the variables.

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References

Linked issue: [Docs] Add target availability connectivity tro... (#577996) • Hannah Baker • 18.6

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Related to #577996