Add cloudstack_instance_group resource

[msinhore]

What

Adds the cloudstack_instance_group resource (create/read/update/delete via the CloudStack instance-group APIs), registered in the provider's ResourcesMap, with acceptance tests and website documentation.

Why

Instance groups let operators organize VMs (and are surfaced by tooling such as the topology designer). The provider previously had no way to manage them.

Files

• cloudstack/resource_cloudstack_instance_group.go (+ test)
• website/docs/r/instance_group.html.markdown
• cloudstack/provider.go (registration)

🤖 Generated with Claude Code

[msinhore]

Heads-up: this PR's failures are not the OpenTofu CI bug — the cloudstack_instance_group acceptance tests fail on all Terraform jobs too. Root cause: the resource creates an empty instance group, but the simulator's listInstanceGroups does not return a group with no instances, so the post-create Read clears the ID ("Provider produced inconsistent result after apply"); the _update/_import cases then hit "group already exists" from the leaked group.

Instance groups in CloudStack are normally created implicitly by deploying a VM with a group attribute, so a standalone empty-group resource is awkward to test on the simulator. Suggest holding this PR (draft/close) and revisiting with an acceptance test that deploys a VM into the group, in an environment where it can be validated. The other PRs (#2–#5) don't depend on it.