docs: add reference for Voellmy-type shallow flow model

docs/source/refs.bib

+@article{Christen2010,
+  title = {{RAMMS}: Numerical simulation of dense snow avalanches in three-dimensional terrain},
+  journal = {Cold Regions Science and Technology},
+  volume = {63},
+  number = {1},
+  pages = {1--14},
+  year = {2010},
+  doi = {https://doi.org/10.1016/j.coldregions.2010.04.005},
+  author = {M. Christen and J. Kowalski and P. Bartelt}
+}
+
+@article{Fischer2012,
+  title = "Topographic curvature effects in applied avalanche modeling",
+  journal = "Cold Regions Science and Technology",
+  volume = "74-75",
+  pages = "21--30",
+  year = "2012",
+  doi = "https://doi.org/10.1016/j.coldregions.2012.01.005",
+  author = "Jan-Thomas Fischer and Julia Kowalski and Shiva P. Pudasaini"
+}
+
 @article{Gu2016,
   title = "Parallel partial {Gaussian} process emulation for computer models with massive output",
   journal = "Annals of Applied Statistics",
@@ -31,6 +52,17 @@
   author = "Meng Yang Gu and Jesus Palomo and James O. Berger",
 }
 
+@article{Mergili2017,
+  AUTHOR = {Mergili, M. and Fischer, J. T. and Krenn, J. and Pudasaini, S. P.},
+  TITLE = {r.avaflow v1, an advanced open-source computational framework for the propagation and interaction of two-phase mass flows},
+  JOURNAL = {Geoscientific Model Development},
+  VOLUME = {10},
+  YEAR = {2017},
+  NUMBER = {2},
+  PAGES = {553--569},
+  DOI = {10.5194/gmd-10-553-2017}
+}
+
 @article{Zhao2021a,
   title   = "Emulator-based global sensitivity analysis for flow-like landslide run-out models",
   journal = "Landslides",