docs: write SymbolCache tutorial

Author: AayushSabharwal

docs/src/simple_sii_sys.md

@@ -4,3 +4,78 @@ In this tutorial we will show how to implement a system structure type for defin
 symbolic indexing of a domain-specific language. This tutorial will show how the
 `SymbolCache` type is defined to take in arrays of symbols for its independent, dependent,
 and parameter variable names and uses that to define the symbolic indexing interface.
+
+## Defining the ODE
+
+For this example, we will use the Robertson equations:
+
+```math
+\begin{aligned}
+\frac{dy_1}{dt} &= -0.04y₁ + 10^4 y_2 y_3 \\
+\frac{dy_2}{dt} &= 0.04 y_1 - 10^4 y_2 y_3 - 3*10^7 y_{2}^2 \\
+\frac{dy_3}{dt} &= 3*10^7 y_{2}^2 \\
+\end{aligned}
+```
+
+The in-place function for this ODE system can be defined as:
+
+```@example symbolcache
+function rober!(du, u, p, t)
+    y₁, y₂, y₃ = u
+    k₁, k₂, k₃ = p
+    du[1] = -k₁ * y₁ + k₃ * y₂ * y₃
+    du[2] = k₁ * y₁ - k₂ * y₂^2 - k₃ * y₂ * y₃
+    du[3] = k₂ * y₂^2
+    nothing
+end
+```
+
+To add symbolic names for the states in this example, a [`SymbolCache`](@ref) can be
+created and passed as the `sys` keyword argument to the `ODEFunction` constructor,
+as shown below:
+
+```@example symbolcache
+using OrdinaryDiffEq, SymbolicIndexingInterface
+
+sys = SymbolCache([:y₁, :y₂, :y₃])
+odefun = ODEFunction(rober!; sys = sys)
+nothing # hide
+```
+
+This is then used to create and solve the `ODEProblem`
+
+```@example symbolcache
+prob = ODEProblem(odefun, [1.0, 0.0, 0.0], (0.0, 1e5), [0.04, 3e7, 1e4])
+sol = solve(prob, Rosenbrock23())
+```
+
+The solution can now be indexed symbolically:
+
+```@example symbolcache
+sol[:y₁]
+```
+
+```@example symbolcache
+sol(1e3, idxs=:y₁)
+```
+
+However, we did not give names to the parameters or the independent variables. They can
+be specified using `SymbolCache` as well:
+
+```@example symbolcache
+sys = SymbolCache([:y₁, :y₂, :y₃], [:k₁, :k₂, :k₃], :t)
+odefun = ODEFunction(rober!; sys = sys)
+prob = ODEProblem(odefun, [1.0, 0.0, 0.0], (0.0, 1e5), [0.04, 3e7, 1e4])
+sol = solve(prob, Rosenbrock23())
+getk1 = getp(sys, :k₁)
+
+getk1(prob)
+```
+
+```@example symbolcache
+getk1(sol)
+```
+
+```@example symbolcache
+sol[:t]
+```