fix: Fixes Getting Started tutorial

[Gmerold]

Description

Fixes Getting Started tutorial. This change reflects the fact that the Terraform modules expose external relations as Juju offers. As a result, the RAN part of the tutorial has been moved to a separate Juju model.

Checklist:

• [x] Documentation follows the Canonical Documentation Style Guide
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [x] I validated that new and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library