Docstrings for PyMaterialXCore classes.

[StefanHabel]

Mostly copied from corresponding C++ header files, but using backticks to automatically link to relevant sections in the Python API documentation, e.g. Document to link to PyMaterialXCore.Document.html.

Using mod.attr("<name>").doc() = ... syntax so that this patch only adds new lines, rather than modifying existing lines.

Using :see: fields to link to corresponding C++ API docs pages.

The docstrings can be retrieved using help(), e.g.

$ python3 -c 'import PyMaterialXCore; help(PyMaterialXCore.NodeDef)'

Help on class NodeDef in module PyMaterialXCore:

class NodeDef(InterfaceElement)
 |  A node definition element within a `Document`.
 |
 |  A `NodeDef` provides the declaration of a node interface, which may then
 |  be instantiated as a `Node`.
 |
 |  :see: https://materialx.org/docs/api/class_node_def.html
 |
 |  Method resolution order:
 |      NodeDef
 |      InterfaceElement
 |      TypedElement
 |      Element
 |      pybind11_builtins.pybind11_object
 |      builtins.object
 |
 |  Methods defined here:
 |
 |  __init__(self, /, *args, **kwargs)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |
[...]

Demo of generated HTML page available here:
https://stefanhabel.github.io/generated/PyMaterialXCore.html

Here's how the above docstring of the NodeDef class appears in the Python API documentation:
https://stefanhabel.github.io/generated/PyMaterialXCore.NodeDef.html

Before	After

Split from #1567.

Update #342.