docs(pcluster): install AWS CLI v2 via official installer instead of pip (#1213)

* docs(pcluster): install AWS CLI v2 via official installer, not pip

pip only distributes AWS CLI v1; installing awscli inside the pcluster
venv silently pinned users to the legacy v1. aws-parallelcluster depends
on boto3, not awscli, so the CLI is only needed for 'aws configure'.
Move AWS CLI v2 install outside the venv using the official installer.

* docs(pcluster): link to official AWS CLI install guide instead of inline installer

Replace the Linux-only curl/unzip snippet with a link to the official
'Install or update to the latest version of the AWS CLI' guide, which
covers Linux, macOS, and Windows and stays current without doc drift.