Ad intro to Julia references

Author: TorkelE

docs/src/introduction_to_catalyst/catalyst_for_new_julia_users.md

@@ -185,3 +185,8 @@ plot(sol)
 ---
 ## Feedback
 If you are a new Julia user who has used this tutorial, and there was something you struggled with or would have liked to have explained better, please [raise an issue](https://github.com/SciML/Catalyst.jl/issues). That way, we can continue improving this tutorial.
+
+---
+## References
+[^1]: [Jeff Bezanson, Alan Edelman, Stefan Karpinski, Viral B. Shah, *Julia: A Fresh Approach to Numerical Computing*, SIAM Review (2017).](https://epubs.siam.org/doi/abs/10.1137/141000671)
+[^2]: [Torkel E. Loman, Yingbo Ma, Vasily Ilin, Shashi Gowda, Niklas Korsbo, Nikhil Yewale, Chris Rackauckas, Samuel A. Isaacson, , *Catalyst: Fast and flexible modeling of reaction networks*, PLOS Computational Biology (2023).](https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1011530)

docs/src/introduction_to_catalyst/introduction_to_catalyst.md

@@ -346,3 +346,7 @@ and the ODE model
    [`ODEProblem`s](https://mtk.sciml.ai/dev/systems/ODESystem/#DiffEqBase.ODEProblem)
    and
    [`SDEProblem`s](https://mtk.sciml.ai/dev/systems/SDESystem/#DiffEqBase.SDEProblem).
+
+---
+## References
+[^1]: [Torkel E. Loman, Yingbo Ma, Vasily Ilin, Shashi Gowda, Niklas Korsbo, Nikhil Yewale, Chris Rackauckas, Samuel A. Isaacson, , *Catalyst: Fast and flexible modeling of reaction networks*, PLOS Computational Biology (2023).](https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1011530)