Add FDTallyGradAux docs.

Ref. neams-th-coe#1031

doc/content/bib/cardinal.bib

@@ -232,3 +232,13 @@ @inproceedings{eltawila
 booktitle = {Proceedings of Physor},
      year = 2024
 }
+
+@article{stolte,
+  title = {{χ-MeRA}: Computationally efficient adaptive mesh refinement of Monte Carlo mesh based tallies},
+  journal = {Annals of Nuclear Energy},
+  volume = {182},
+  pages = {109617},
+  year = {2023},
+  doi = {https://doi.org/10.1016/j.anucene.2022.109617},
+  author = {K. N. Stolte and P. V. Tsvetkov}
+}

doc/content/source/auxkernels/FDTallyGradAux.md

@@ -0,0 +1,27 @@
+# FDTallyGradAux
+
+## Description
+
+`FDTallyGradAux` approximates tally gradients using forward finite differences between the current element and it's neighbors.
+The approach used is based on the derivative approximation presented in [!cite](stolte):
+
+\begin{equation}
+\label{tally_grad}
+\vec{\nabla}u(\vec{x}_{i}) \approx Y_{i}^{-1}\sum_{i'} \frac{\vec{x}_{i'} - \vec{x}_{i}}{||\vec{x}_{i'} - \vec{x}_{i}||}\frac{u(\vec{x}_{i'}) - u(\vec{x}_{i})}{||\vec{x}_{i'} - \vec{x}_{i}||}\text{,}
+\end{equation}
+\begin{equation}
+\label{tally_grad_Y}
+Y_{i} = \sum_{i'} (\vec{x}_{i'} - \vec{x}_{i})\otimes (\vec{x}_{i'} - \vec{x}_{i}) \text{,}
+\end{equation}
+where $i$ indicates the current element, $i'$ is a neighboring element, $\vec{x}$ is an element's centroid, and $u$ is the tally field. A tally `score` must be specified, and if any external filter bins have been added through the use of the [filter system](AddFilterAction.md) `ext_filter_bin` index should be provided.
+
+## Example Input Syntax
+
+This example showcases how `FDTallyGradAux` can be used to approximate the gradient of `kappa_fission`:
+
+!listing test/tests/neutronics/tally_grad/finite_diff/openmc.i
+  block=AuxKernels
+
+!syntax parameters /AuxKernels/FDTallyGradAux
+
+!syntax inputs /AuxKernels/FDTallyGradAux