Remove Potjans and Diesmann microcircuit example from this repo and redirect to new one

[jessica-mitchell]

The microcircuit model has its own repository and corresponding Read the Docs page:

• https://github.com/INM-6/microcircuit-PD14-model/tree/main
• https://microcircuit-pd14-model.readthedocs.io/en/latest/index.html

This PR removes the example from nest simulator and updates the links using intersphinx to the new Read the Docs documentation pages

The documentation build can be found in the workflows below (checks). Click on the docs/readthedocs link

[tomtetzlaff]

Thanks Jessica. This looks good to me.