Update doc for Autoscaling GitLab CI on AWS Fargate, adds troubleshooting section.

What does this MR do?

Enhancement to documentation, adds a troubleshooting section.

Why was this MR needed?

Issues have been submitted pertaining to this feature.

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

gitlab-org/ci-cd/custom-executor-drivers/fargate#22 (closed)

gitlab-org/ci-cd/custom-executor-drivers/fargate!40 (closed)