Add notes about external services in static inventory and about ARM for machine images

What does this MR do?

• Add note about external PG/Redis usage for static inventory
• Add notes about ARM for machine images
• Smaller clean ups in docs, such as removing external_ip from AWS example to align with https://gitlab.com/gitlab-org/gitlab-environment-toolkit/-/blob/main/docs/environment_advanced_hybrid.md#amazon-web-services-aws-1

Related issues

Closes #1061 (closed) #1036 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gitlab-org/software-delivery/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.