Docu: Add example on how to realize port display in needflow

[StephanKuempel]

When using style="portin", "portout" or "port" for a needs_type and using nested needs to put the ports under another element (e.g. a block) a needflow covering those elements will display them nicely. Additionally a needs_diagram_template for the port to like this

needs_diagram_template = """
{%- if (type_name == "Publisher Port") -%}
<size:12>{{type}}</size>
...

to only to display the title of the port is very helpful to clean up clutter in the graph.

Maybe a scenario like this could be added to the documentation, since the usage of ports is quite common (e.g. in SysML)