standardize citation in the documentation

[Remi-Gau]

It seems that in several examples (and perhaps in the rest of the doc: I have not checked), citaions to papers are made using very different "formats".

It would probably be better to use the sphinxcontrib-bibtex machinery used in other part of the doc to render this in a more consistent manner and to make it easier to hyperlink to the original papers.

One example of a "verbatim" citation (out of several), that will force the user to copy paste the reference into a search engine.

nilearn/examples/00_tutorials/plot_decoding_tutorial.py

Line 9 in 6764c9e

* J.V. Haxby et al. "Distributed and Overlapping Representations of Faces

https://nilearn.github.io/stable/auto_examples/00_tutorials/plot_decoding_tutorial.html#a-introduction-tutorial-to-fmri-decoding