Fix documentation, remove version cap on sphinxcontrib-bibtex

[Moritz-Alexander-Kern]

This PR removes the version cap on sphinxcontrib-bibtex==1.0.0.

Newer versions of sphinxcontrib-bibtex require the path to the .bib file to be specified in the configuration file for the documentation (conf.py).

• removed version cap for sphinxcontrib-bibtex
• added path to viziphant.bib file to conf.py

See docs here: https://viziphant-experimental.readthedocs.io/en/fix-bibtex/modules.html

[Moritz-Alexander-Kern]

todo:

• [x] add new citation style, analogous to https://github.com/NeuralEnsemble/elephant/pull/523