Bifurcationkit periodic orbits example

[TorkelE]

Adds an example of how to compute the periodic orbits of an oscillator system using BifurcationKit.

[rveltz]

I meant https://bifurcationkit.github.io/BifurcationKitDocs.jl/dev/#Citing-this-work

The paragraph "citing this work"

[TorkelE]

New ending

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## [Citation](@id bifurcationkit_periodic_orbits_citation)
If you use BifurcationKit.jl for your work, we ask that you **cite** the following paper!! Open source development strongly depends on this. It is referenced on [HAL-Inria](https://hal.archives-ouvertes.fr/hal-02902346) with *bibtex* entry [CITATION.bib](https://github.com/bifurcationkit/BifurcationKit.jl/blob/master/CITATION.bib).