Docs: Refactor "cluster levels"

Follows conversation with Viktor about dropping the reference to "cluster levels".
Follows !71588 (comment 698050763).

Reword what we call "cluster levels" as this is just a legacy term used internally at GitLab. The term is not part of the industry terminology.

Remove the 3 pages (project level, group level, instance level).
Highlight only that the user can connect a cluster in different "scopes" (should we use this term?) meant for different interactions with GitLab:
- Connect to a project - to use the cluster with one single project.
- Connect to a group - to use the cluster in projects within a group.
- Connect to an instance - to use the cluster in all projects within the instance.
Indicate the availability of each feature according to each scope.

~"devops::configure"

ETA: %14.5 ish