Provide documentation about when to create a new node type

[oxisto]

Maybe place here: https://github.com/Fraunhofer-AISEC/cpg/blob/main/docs/docs/Contributing/index.md#code-guidelines

Some guidelines from @maximiliankaul. It is ok to add a new node type if:

• It is not possible to otherwise realise this construct
• The concept is known in at least 2 langauges
• It will significantly ease the live of the developers, e.g., in terms of maintainability

[maximiliankaul]

⬆️ just something that came up in another chat. If one of these conditions is true, then we should consider a new node. If non of them are true -> no need for a new node.