Gabeaws/docs/getting started updates

[gabebatista]

Issue number: #355

Summary

Updates documentation for getting started with the simple build pipeline sample and adds missing defaults packer template.

Changes

• added warnings regarding the region variable in packer templates as they default to us-west-2 and can cause errors.
• Changed Packer commands to proper syntax in getting started docs
• Changed the Note admonition in the Amazon Linux 2023 ARM based Amazon Machine Image section to a warning, as not heading that admonition would cause the command to fail.
• Added defaults to the vpc_id and subnet_id variables in amazon-linux-2023-arm64.pkr.hcl, allowing Packer to default to the default VPC if no value is passed in.
• Renamed the fully_qualified_domain_name reference inStep 5. Configure Simple Build Pipeline Variables to root_domain_name to align with name change in code.
• Split Step 5. Configure Simple Build Pipeline Variables section into local.tf and variables.tf paragraphs to align with module structure.

User experience

Fixes faulty code and clarifies sections which are most likely to cause issues for users.

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Is this a breaking change?

No

Acknowledgment

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[henrykie]

closes #355 @gabebatista ?

Addressed in comment.