feat: TELCO-939 Cups tutorial with Multipass

[gatici]

Description

This PR adds how to deploy and run the SD-Core 5G core network in a single server following Control and User Plane Separation (CUPS) principles using Multipass and LXD backend.

Checklist:

• [x] My code follows the style guidelines of this project
• [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
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library

[Gmerold]

Why are we changing the existing tutorial instead of creating a new one? Seems to me that both cases are valid, aren't they?

[gatici]

I think it would be good to unify the spelling across this file, i.e.:

* LXD (product) vs lxd (command)

* IP (protocol abbreviation) vs ip (command)
  etc

Done

[gatici]

Why are we changing the existing tutorial instead of creating a new one? Seems to me that both cases are valid, aren't they?

The existing guide does not include the steps about infrastructure preparation such as VM creation, adding routes and configuring the DNS. This PR includes the steps all along.