[doc] spinning up again

doc/index.rst

@@ -10,4 +10,5 @@ Welcome to the Calibr8 documentation.
     :maxdepth: 1
     :caption: Table of Contents:
 
+    theory/theory
     usage/usage

doc/theory/theory.rst

@@ -0,0 +1,27 @@
+Theory
+======
+
+Governing Equations
+-------------------
+
+At its core, CALIBR8 is concerned with solving the balance of linear momentum in the
+absence of inertial terms for finite-deformation mechanics in a total-Lagrangian setting.
+This can be represented as
+
+.. math::
+
+  \begin{aligned}
+  \begin{cases}
+  - \nabla \cdot \boldsymbol{P} &= 0, &&\text{in} \quad \mathcal{B}, \\
+  \boldsymbol{u} &= \boldsymbol{g}, &&\text{on} \quad \Gamma_g, \\
+  \boldsymbol{P} \cdot \boldsymbol{N} &= \boldsymbol{h}, &&\text{on} \quad \Gamma_h.
+  \end{cases}
+  \end{aligned}
+
+where
+:math:`\boldsymbol{P}` denotes the first Piola-Kirchhoff stress tensor
+:math:`\mathcal{B}` denotes the domain of interest in the reference configuration,
+:math:`\Gamma_g` denotes the portion of the domain boundary on which Dirichlet boundary
+conditions :math:`\boldsymbol{u}` are prescribed, and :math:`\Gamma_h` denotes the portion
+of the domain boundary on which tractions
+:math:`\boldsymbol{P} \cdot \boldsymbol{N}` are prescribed,