Update the AWS Installation documentation

- Move the existing manual install instructions page to a subpage
  and mark it as DEPRECATED in order to clear the 'install/aws'
  URL to be an index to all AWS documentation.
- Created a new 'install/aws/index.md'
  - to give an overview of how the AWS opinionated documentation
    relates to and builds upon the reference architectures.
  - to have an index of sub-pages at the bottom.
- Provides a new sub page for GitLab Cloud Native Hybrid on AWS EKS,
  which provides critical Bill Of Materials lists for the upcoming
  AWS Quick Start for GitLab Cloud Native on EKS.