Added missing doxygen bibliography file.

docs/html/doxygen/doxygen.bib

+@article{CaYaKiPeSh13,
+  title={High-order spectral/< i> hp</i> element discretisation for reaction-diffusion problems on surfaces: Application to cardiac electrophysiology},
+  author={Cantwell, CD and Yakovlev, S and Kirby, RM and Peters, NS and Sherwin, SJ},
+  journal={Journal of Computational Physics},
+  year={2013},
+  publisher={Elsevier}
+}