docs: Uses charmed 5G specific networking content

[gruyaume]

Description

It is hard to wrap one's head around the various networks that need to be created when deploying SD Core, especially in control plane / user plane separated contexts. The existing diagram and content is a generic 5G diagram and is not useful when deploying SD-Core. This PR improves the current content by adding a list of networks that need to be created and removing the generic 5G content that can already be found elsewhere in the interwebs.

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] 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