Resolve "Update SAST doc and example"

What does this MR do?

Update all documentation related to SAST.

Are there points in the code the reviewer needs to double check?

No. Only the documentation changes.

Why was this MR needed?

Because the doc doesn't reflect what's in %10.5.

Screenshots (if relevant)

Not relevant.

Does this MR meet the acceptance criteria?

(https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)

• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #4845 (closed)