Rework documentation for atifacts and transformations

[wvandeun]

Recently a requirement was introduced for schemanodes to inherit from the CoreArtifactTarget generic, if you want to be able to generate artifacts for them.

To capture this in the documentation we had to redo the artifact and transformation guides. They were leveraging the BuiltinTag object to avoid having to load a custom schema into Infrahub. But a user cannot modify the schema of a BuiltinTag. Therefor we had to redo this documentation.

[BeArchiTek]

LGTM. I was confused about creating a newNetworkDevice model instead of editing the one in the demo-schema, before realizing this was meant to be a "standalone" guide.