Provide bibtex for citing Nengo and NEF

[arvoelke]

This issue is motivated by:

• https://forum.nengo.ai/t/howto-cite-nengo/991
• The NEF text is still frequently cited incorrectly with the year 2004, since this is what google scholar uses for the top version.

We could provide Nengo+NEF citation instructions here for example: https://www.nengo.ai/publications/

Alternatively, could locate inside the Nengo core documentation, to mirror what Nengo-DL does here: https://www.nengo.ai/nengo-dl/project.html#citation