add Prev and Next to docs

Author: vyudu

docs/src/systems/DiscreteSystem.md

@@ -26,3 +26,10 @@ structural_simplify
 DiscreteProblem(sys::DiscreteSystem, u0map, tspan)
 DiscreteFunction(sys::DiscreteSystem, args...)
 ```
+
+## Discrete Domain
+```@docs; canonical=false
+Shift
+Prev
+Next
+```

docs/src/systems/ImplicitDiscreteSystem.md

@@ -26,3 +26,10 @@ structural_simplify
 ImplicitDiscreteProblem(sys::ImplicitDiscreteSystem, u0map, tspan)
 ImplicitDiscreteFunction(sys::ImplicitDiscreteSystem, args...)
 ```
+
+## Discrete Domain
+```@docs; canonical=false
+Shift
+Prev
+Next
+```

docs/src/tutorials/SampledData.md

@@ -25,8 +25,10 @@ The operators [`Sample`](@ref) and [`Hold`](@ref) are thus providing the interfa
 The [`ShiftIndex`](@ref) operator is used to refer to past and future values of discrete-time variables. The example below illustrates its use, implementing the discrete-time system
 
 ```math
-x(k+1) = 0.5x(k) + u(k)
-y(k) = x(k)
+\begin{align}
+    x(k+1) &= 0.5x(k) + u(k) \\ 
+    y(k) &= x(k)
+\end{align}
 ```
 
 ```@example clocks
@@ -187,3 +189,10 @@ connections = [r ~ sin(t)          # reference signal
 
 @named cl = ODESystem(connections, t, systems = [f, c, p])
 ```
+
+```@docs
+Sample
+Hold
+ShiftIndex
+Clock
+```

src/ModelingToolkit.jl

@@ -301,7 +301,7 @@ export debug_system
 #export ContinuousClock, Discrete, sampletime, input_timedomain, output_timedomain
 #export has_discrete_domain, has_continuous_domain
 #export is_discrete_domain, is_continuous_domain, is_hybrid_domain
-export Sample, Hold, Shift, ShiftIndex, sampletime, SampleTime
+export Sample, Hold, Shift, ShiftIndex, sampletime, SampleTime, Next, Prev
 export Clock, SolverStepClock, TimeDomain
 
 export MTKParameters, reorder_dimension_by_tunables!, reorder_dimension_by_tunables