Adding a docstring for PyMaterialXCore.

This PR adds a docstring in markdown format to the `PyMaterialXCore` module.

The docstring is defined in a macro named `PyMaterialXCore_DOCSTRING` that
uses the new `PYMATERIALX_DOCSTRING` macro to allow us to place the first
and last lines in lines by themselves, rather than starting the docstring
contents immediately after the `R"docstring(` marker.

The `PyMaterialXCore_DOCSTRING` macro is placed in a separate header file
named `__doc__.md.h` which lives side-by-side with the `PyModule.cpp` file
in which it is included.

Split from AcademySoftwareFoundation#1567.

Depends on AcademySoftwareFoundation#2038.

Update AcademySoftwareFoundation#342.

Signed-off-by: Stefan Habel <19556655+StefanHabel@users.noreply.github.com>

source/PyMaterialX/PyMaterialXCore/PyModule.cpp

@@ -4,6 +4,7 @@
 //
 
 #include <PyMaterialX/PyMaterialX.h>
+#include "__doc__.md.h"
 
 namespace py = pybind11;
 
@@ -26,7 +27,7 @@ void bindPyVariant(py::module& mod);
 
 PYBIND11_MODULE(PyMaterialXCore, mod)
 {
-    mod.doc() = "Module containing Python bindings for the MaterialXCore library";
+    mod.doc() = PyMaterialXCore_DOCSTRING;
 
     bindPyElement(mod);
     bindPyTraversal(mod);