Documentation for Group-level Kubernetes cluster configuration

What does this MR do?

Adds documentation about support for group level clusters

• Add docs about how env scope and group clusters work
• Add docs about RBAC limitation for group clusters
• Add docs about how to run Auto DevOps on group clusters - same as per project + how to add AUTO_DEVOPS_DOMAIN on group CI variable
• Document why KUBE_TOKEN could be nil; how to check logs (moved into https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23414)

What are the relevant issue numbers?

#34758 (closed)

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards