Nilearn: Use bibtex in nilearn/signal.py

[NicolasGensollen]

Convert references in nilearn/signal.py to use footcite / footbibliography.
See #2759

[JLefortBesnard]

Hello,
I would like to be assigned on this issue to train more with PR and git commands.
Thank you :)

[JLefortBesnard]

Thank you @NicolasGensollen
Actually, I don't see any reference to add in nilearn/signal.py, or maybe should I create a reference section in the clean() function for Lindquist et al. (2018)?

[bthirion]

In the dostrings there are refs to Behzadi et al. 2007 and Lindquist et al. 2018.