Add documentation for a notation model use case

[ndoschek]

Elaborate the notation model use case (as a decorator model scenario) and show the different possibilities to achieve notation model specific support:

• Store notation related data in the user session (e.g. viewport settings)
• Use a generic map of notation related properties in the notation model (see https://github.com/eclipse-glsp/glsp-server/tree/master/plugins/org.eclipse.glsp.server.emf/model)
• Use a custom notation model

This should be placed in a separate section of the GLSP documentation with links to the corresponding examples. In a further step, those sections could also provide recipes for the mentioned use cases.