[NEST Users] Restructured documentation build system: please update open PRs!

Dear NEST Developers! With the merge of #1843 a few moments ago, we have implemented a significant change in the way NEST documentation is built (thank you, Jochen!). Please merge the current master in all open PRs to avoid complications. With the new setup, documentation can be built with `make doc` and will be built in the build directory and can also be installed. To build the documentation, you will need a range of Python packages. Please see doc/requirements.txt for what is needed. We will continue to make improvements to the documentation build system, but these will be minor. If you have any open PRs, please merge the current master as soon as possible into your branches. Best regards, Hans Ekkehard -- Prof. Dr. Hans Ekkehard Plesser Head, Department of Data Science Faculty of Science and Technology Norwegian University of Life Sciences PO Box 5003, 1432 Aas, Norway Phone +47 6723 1560 Email hans.ekkehard.plesser@nmbu.no<mailto:hans.ekkehard.plesser@nmbu.no> Home http://arken.nmbu.no/~plesser