Validate documentation for ssh key generation and add necessary clarifications

As per https://github.com/aws/eks-anywhere/issues/5420, we want to make sure that we have the proper documentation which describe the behavior during upgrade regarding ssh key validation where empty keys can't be specified and telling users to save the ssh key from create, otherwise they won't be able to access their cluster properly.

Start by reading the behavior described in issue 5420, then locate the EKS Anywhere documentation covering cluster upgrades and SSH key handling. Verify the documentation explains that empty SSH keys cannot be specified and that users must save the key produced during cluster creation; done means those clarifications are present and accurate.

Written by the indexing model from the issue text.