Add object documentation and make website more navigable for basic do…

…cumentation review.

Refs idaholab#25 , idaholab#24

modules/electromagnetics/doc/content/bib/elk.bib

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+@article{cincotti2007sps,
+author = {Cincotti, A. and Locci, A. M. and Orrù, R. and Cao, G.},
+title = {Modeling of {SPS} apparatus: Temperature, current and strain distribution with no powders},
+journal = {AIChE Journal},
+volume = {53},
+number = {3},
+pages = {703-719},
+doi = {10.1002/aic.11102},
+year = {2007}
+}
+
+@article{babu2001contactresistance,
+author = {Babu, S. S. and Santella, M. L. and Feng, Z. and Riemer, B. W. and Cohron, J. W.},
+title = {Empirical model of effects of pressure and temperature on electrical contact resistance of metals},
+journal = {Sci Technol Weld Joining},
+volume = {6},
+number = {3},
+pages = {126-132},
+year = {2001}
+}

modules/electromagnetics/doc/content/source/interfacekernels/ElectrostaticContactCondition.md

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+# ElectrostaticContactCondition
+
+!syntax description /InterfaceKernels/ElectrostaticContactCondition
+
+## Description
+
+This interface kernel models the conductivity of electric field across a specified
+boundary between two dissimilar materials, as described by [!citep](cincotti2007sps).
+It accounts for the influence of electrostatic potential differences across the
+interface, with appropriate electrical contact conductance / resistance coefficient
+being provided either by the user as a constant scalar number or via a combination
+of material properties and constants for calculation. The condition being applied is:
+
+\begin{equation}
+  \sigma_{el,1} \frac{\partial \phi}{\partial \mathbf{r}} \bigg\rvert_1 \cdot \mathbf{\hat{n}} = \sigma_{el,2} \frac{\partial \phi}{\partial \mathbf{r}} \bigg\rvert_2 \cdot \mathbf{\hat{n}}
+\end{equation}
+
+and
+
+\begin{equation}
+  \sigma_{el,1} \frac{\partial \phi}{\partial \mathbf{r}} \bigg\rvert_1 \cdot \mathbf{\hat{n}} = -C_E (\phi_1 - \phi_2)
+\end{equation}
+
+where
+
+- $\sigma_{el, i}$ is the electrical conductivity of each material along the interface,
+- $C_E$ is the electrical contact conductance, and
+- $\phi_i$ is the electrostatic potential of the material at the interface.
+
+The temperature- and mechanical-pressure-dependent electrical contact conductance, given by [!citep](babu2001contactresistance), is calculated using:
+
+\begin{equation}
+  C_E(T, P) = \alpha_E \sigma_{el,Harm} \bigg( \frac{P}{H_{Harm}} \bigg)^{\beta_E}
+\end{equation}
+
+where
+
+- $\alpha_E$ is an experimentally-derived proportional fit parameter (set to be 64, from [!citep](cincotti2007sps)),
+- $\sigma_{el,Harm}$ is the harmonic mean of the temperature-dependent electrical conductivities on either side of the boundary,
+- $P$ ($=F/S$) is the uniform mechanical pressure applied at the contact surface area (S) between the two materials,
+- $H_{Harm}$ is the harmonic mean of the hardness values of each material, and
+- $\beta_E$ is an experimentally-derived power fit parameter (set to be 0.35, from [!citep](cincotti2007sps)).
+
+For reference, the harmonic mean calculation for two values, $V_a$ and $V_b$, is given by
+
+\begin{equation}
+  V_{Harm} = \frac{2 V_a V_b}{V_a + V_b}
+\end{equation}
+
+## Example Input File Syntax
+
+!listing contact_resistance_calculated.i block=InterfaceKernels/contact_resistance
+
+
+!syntax parameters /InterfaceKernels/ElectrostaticContactCondition
+
+!syntax inputs /InterfaceKernels/ElectrostaticContactCondition
+
+!syntax children /InterfaceKernels/ElectrostaticContactCondition