[docs] Specify core vs saas features more clearly

[adnanrahic]

• Add callouts on pages that explain which features are free/paid.
• Add lists with detailed capabilities and links to the pages explaining them
• Explain feature differences for core vs saas on a dedicated page

[adnanrahic]

Also add a callout to pages that outline paid features like this: https://docs.tracetest.io/concepts/run-groups