Add EC2 SAST dev machine setup instructions

What does this MR do?

@jamesliu-gitlab has created a SAST development image in EC2 to help engineers overcome any issues they might be having running SAST analyzers on an M1 mac. This MR moves the setup instructions from slack which will be lost in 90 days to a longer lasting location.

What are the relevant issue numbers?

NA

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer