Added documentation for central difference timeintegrator. Refs idaho…

…lab#13964.

framework/doc/content/source/timeintegrators/CentralDifference.md

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+# CentralDifference
+
+!syntax description /Executioner/TimeIntegrator/CentralDifference
+
+## Description
+
+Central difference integration is a common explicit integration scheme, typically used for structural dynamics problems. For a variable, $u$, at time $t$ with a time step, $\Delta t$, the central difference approximations for the first and second time derivatives, $\dot{u}$ and $\ddot{u}$, are given as,
+
+\begin{equation}
+\begin{aligned}
+\mathbf{\dot{u}}(t) &=& \frac{\mathbf{u}(t+\Delta t)-\mathbf{u}(t-\Delta t)}{2 \Delta t} \\
+\mathbf{\ddot{u}}(t) &=& \frac{\mathbf{u}(t+\Delta t)-2\mathbf{u}(t)+\mathbf{u}(t-\Delta t)}{\Delta t^2}
+\end{aligned}
+\end{equation}
+
+The central difference time integrator derives from the [`ActuallyExplicitEuler`](ActuallyExplicitEuler.md) class and therefore circumvents the nonlinear solver. It can be used with `consistent`, `lumped` or `lump_preconditioned`, `solve_type` options.
+
+## Assumptions
+
+The implementation of the central difference timeintegrator assumes that corresponding kernels involving the first and second derivatives directly use the derivatives calculates by the timeintegrator. Currently, only the `InertialForce` kernel and the `NodalKernels` corresponding to nodal inertias are modified to work with the central difference timeintegrator. Therefore, the central difference time integrator can currently only be used for mechanics problems with solid elements. 
+
+!syntax parameters /Executioner/TimeIntegrator/CentralDifference
+
+!syntax inputs /Executioner/TimeIntegrator/CentralDifference
+
+!syntax children /Executioner/TimeIntegrator/CentralDifference