docs: add region configuration note for SageMaker deployment

[liuyuelintop]

Problem

Students deploying to regions other than us-east-1 may encounter a cross-region ECR error during the SageMaker deployment step.
This happens because the sagemaker_image_uri variable in terraform/2_sagemaker/variables.tf has a hardcoded us-east-1 in its default value.

Error message:

ValidationException: Region of "763104351884.dkr.ecr.us-east-1.amazonaws.com/..."
does not match expected region of ap-southeast-2.
Cross region ECR image pulls are not allowed.

---

Solution

Added a clear note and warning in Guide 2 (Step 1), right after the terraform.tfvars configuration section, to help students update the region in variables.tf before running terraform apply.

The note includes:

• ⚠️ Warning icon for better visibility
• File and line reference (variables.tf, line 9)
• Simple, step-by-step instructions
• Example using another region (ap-southeast-2)
• A short explanation of why this update is necessary

---

Testing

Verified in the ap-southeast-2 region — following these new instructions successfully prevents the deployment error.

---

Impact

• 🧩 Non-breaking change — documentation only
• Reduces confusion and failed deployments for students outside us-east-1
• Improves learning flow in Guide 2
• Can be complemented later with a code-level fix for full region-agnostic support