Write project structure section

Author: Steven-Roberts

docs/structure/index.rst

@@ -1,13 +1,43 @@
 Project Structure
-================================================================================
+=================
 
-TODO
+The ODE Test Problems API resides under the ``otp`` namespace. At the top level of this namespace, there are three
+classes that serve as the foundation for all problem implementations:
 
 .. toctree::
-   :caption: Top-Level Classes
-   :maxdepth: 1
-   :glob:
 
    problem
    rhs
    parameters
+
+Each problem in OTP defines a sub-namespace of ``otp`` with a standard structure:
+
+::
+
+   otp.<problemname>
+   ├─ <ProblemName>Problem.m
+   ├─ <ProblemName>Parameters.m
+   └─ presets
+      ├─ Canonical.m
+      ├─ <PresetName>.m
+      ├─ <AnotherPresetName>.m
+      └─ ...
+
+This allows a problem to be manually instantiated with
+
+>>> timeSpan = ...
+>>> y0 = ...
+>>> parameters = otp.<problemname>.<ProblemName>Parameters('<Param1>', value1, ...);
+>>> problem = otp.<problemname>.<ProblemName>Problem(timeSpan, y0, parameters);
+
+In most cases, it is easier to use a preset instead. A preset is a subclass of ``<ProblemName>Problem.m`` which
+specifies values for the problem time span, initial conditions, and parameters, typically using values proposed in the
+literature. Every problem has a preset named ``Canonical`` which can be constructed with
+
+>>> problem = otp.<problemname>.presets.Canonical;
+
+Constructors for presets can be called without arguments like the code above, but many presets also accept optional
+name-value pairs to override default values. Regardless, parameters can always be modified after instantiating a
+problem:
+
+>>> problem.Parameters.<Param1> = 1234;