Add MFEMScalarBoundaryIntegratedBC source documentation

doc/content/source/bcs/MFEMScalarBoundaryIntegratedBC.md

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+# MFEMScalarBoundaryIntegratedBC
+
+## Summary
+
+!syntax description /BCs/MFEMScalarBoundaryIntegratedBC
+
+## Overview
+
+Adds the boundary integrator for integrating the linear form
+
+!equation
+(f, v)_{\partial\Omega} \,\,\, \forall v \in V
+
+where $v \in H^1$ and $f$ is a scalar coefficient. Often used for representing Neumann-type boundary
+conditions.
+
+!syntax parameters /BCs/MFEMScalarBoundaryIntegratedBC
+
+!syntax inputs /BCs/MFEMScalarBoundaryIntegratedBC
+
+!syntax children /BCs/MFEMScalarBoundaryIntegratedBC