Leverage "subcategory" For Organizing Docs

[trotttrotttrott]

We'd like to leverage subcategories to improve documentation navigation on the Terraform registry. I'd like to propose Grafana, Enterprise, and Cloud as our initial subcategories. This would produce the following navigation hierarchy:

• Grafana
  • Resources
    • grafana_alert_notification
    • grafana_dashboard
    • grafana_dashboard_permission
    • grafana_data_source
    • ...
• Enterprise
  • Resources
    • grafana_builtin_role_assignment
    • grafana_role
• Cloud
  • Resources
    • grafana_synthetic_monitoring_check
    • grafana_synthetic_monitoring_probe
  • Data Sources
    • grafana_synthetic_monitoring_probe
    • grafana_synthetic_monitoring_probes