Clarify how to download sast report artifact in UI

What does this MR do?

Clarifies that artifacts:paths must be set to download gl-sast-report.json artifact via the UI.

Screenshots (strongly suggested)

Without artifacts:paths for gl-sast-report.json

SAST job run with just

include:
  - template: Security/SAST.gitlab-ci.yml

Scans files but artifacts are not downloadable via the UI

With artifacts:paths for gl-sast-report.json

include:
  - template: Security/SAST.gitlab-ci.yml

phpcs-security-audit-sast:
  artifacts:
    paths:
      - gl-sast-report.json

@rdickenson ready for review!

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team