docs: update references

docs/source/refs.bib

+@article{Gu2016,
+  title = "Parallel partial {Gaussian} process emulation for computer models with massive output",
+  journal = "Annals of Applied Statistics",
+  volume = "10",
+  number = "3",
+  pages = "1317--1347",
+  year = "2016",
+  doi = "10.1214/16-AOAS934",
+  author = "Gu, Meng Yang and Berger, James O."
+}
+
+@article{Gu2018,
+  title = "Robust {Gaussian} stochastic process emulation",
+  journal = "Annals of Statistics",
+  volume = "46",
+  number = "6A",
+  pages = "3038--3066",
+  year = "2018",
+  doi = "10.1214/17-AOS1648",
+  author = "Gu, Meng Yang and Wang, Xiao Jing and Berger, James O."
+}
+
+@article{Gu2019,
+  title = "RobustGaSP: Robust {Gaussian} stochastic process emulation in {R}",
+  journal = {{The R Journal}},
+  pages = {112--136},
+  volume = {11},
+  number = {1},
+  year = {2019},
+  doi = {10.32614/RJ-2019-011},
+  author = "Meng Yang Gu and Jesus Palomo and James O. Berger",
+}
+
+@article{Zhao2021a,
+  title   = "Emulator-based global sensitivity analysis for flow-like landslide run-out models",
+  journal = "Landslides",
+  volume  = "",
+  number  = "",
+  pages   = "",
+  year    = "2021",
+  doi     = "https://doi.org/10.1007/s10346-021-01690-w",
+  author  = "Zhao, H. and Amann, F. and Kowalski, J."
+}
+
+@PhdThesis{Zhao2021b,
+  author       = {Zhao, Hu},
+  school       = {RWTH Aachen University},
+  title        = {Gaussian processes for sensitivity analysis, {Bayesian} inference, and uncertainty quantification in landslide research},
+  year         = {2021},
+  doi          = {10.18154/RWTH-2021-11693}
+}