add fixed dt SDE simulation section

Author: TorkelE

docs/src/model_simulation/simulation_introduction.md

@@ -222,6 +222,17 @@ sol = solve(sprob, STrapezoid(); seed = 12345, abstol = 1e-1, reltol = 1e-1) # h
 plot(sol)
 ```
 
+### [SDE simulations with fixed time stepping](@id simulation_intro_SDEs_fixed_dt)
+StochasticDiffEq implements SDE solvers with adaptive time stepping. However, when using a non-adaptive solver (or using the `adaptive = false` argument to turn adaptive time stepping off for an adaptive solver) a fixed time step `dt` must be designated. Here we simulate the same `SDEProblem` which we struggled with previously, but using the non-adaptive [`EM`](https://en.wikipedia.org/wiki/Euler%E2%80%93Maruyama_method) solver and a fixed `dt`:
+```@example simulation_intro_sde
+sol = solve(sprob, EM(); dt = 0.001)
+sol = solve(sprob, EM(); dt = 0.001, seed = 1234567) # hide
+plot(sol)
+```
+We note that this approach also enables us to successfully simulate the SDE we previously struggled with.
+
+Generally, using a smaller fixed `dt` provides a more exact simulation, but also increases simulation runtime.
+
 ### [Scaling the noise in the chemical Langevin equation](@id simulation_intro_SDEs_noise_saling)
 When using the CLE to generate SDEs from a CRN, it can sometimes be desirable to scale the magnitude of the noise. This can be done by introducing a *noise scaling term*, with each noise term generated by the CLE being multiplied with this term. A noise scaling term can be set using the `@default_noise_scaling` option:
 ```@example simulation_intro_sde
@@ -338,9 +349,10 @@ end
 ```
 This type of model will generate so called *variable rate jumps*. Simulation of such model is non-trivial (and Catalyst currently lacks a good interface for this). A detailed description of how to carry out jump simulations for models with time-dependant rates can be found [here](https://docs.sciml.ai/JumpProcesses/stable/tutorials/simple_poisson_process/#VariableRateJumps-for-processes-that-are-not-constant-between-jumps).
 
+
 ---
 ## [Citation](@id simulation_intro_citation)
-When you simulate Catalyst models in your research, please cite the corresponding paper(s) to support the package authors. For ODE simulations:
+When you simulate Catalyst models in your research, please cite the corresponding paper(s) to support the simulation package authors. For ODE simulations:
 ```
 @article{DifferentialEquations.jl-2017,
  author = {Rackauckas, Christopher and Nie, Qing},