Added section describing the MaterialX Python API. (#1567)

...to mention the `MaterialX` Python package, and the way it's typically
aliased as `mx` upon `import`.

Also removed the paragraph about the MaterialX.org site, as it seemed
unnecessary.

Also added a link to the auto-generated alphabetical index to the TOC.

Signed-off-by: Stefan Habel <[email protected]>

documents/DeveloperGuide/index.rst

@@ -1,7 +1,7 @@
 MaterialX Python API Documentation
 ==================================
 
-The MaterialX Python API provides Python bindings for the
+The `MaterialX Python API`_ provides Python bindings for the
 `MaterialX C++ API <https://materialx.org/docs/api/>`_ .
 
 MaterialX is an open standard for representing rich material and look-development
@@ -15,10 +15,6 @@ Adobe, and SideFX contributing to its ongoing development. In 2021, MaterialX
 became the seventh hosted project of the `Academy Software Foundation
 <https://www.aswf.io/>`_ .
 
-General information about MaterialX, announcements, events, third-party support,
-and contributing to MaterialX is available at
-`MaterialX.org <https://materialx.org/>`_ .
-
 .. toctree::
     :maxdepth: 1
 
@@ -28,6 +24,43 @@ and contributing to MaterialX is available at
     ShaderGeneration.md
 
 
+MaterialX Python API
+--------------------
+
+The MaterialX Python API consists of two parts:
+
+- A set of `MaterialX Python Modules`_ that are implemented as Python C
+  extensions that correspond to MaterialX C++ libraries.
+- A Python package named `MaterialX` that wraps the MaterialX Python modules to
+  provide a more pythonic interface, in particular for working with the
+  `Element Classes <generated/PyMaterialXCore.html#element-classes>`_ and
+  `Value Classes <generated/PyMaterialXCore.html#value-classes>`_ of
+  `PyMaterialXCore`.
+
+The `MaterialX` Python package is typically imported aliased as `mx`:
+
+.. code:: python
+
+   import MaterialX as mx
+
+All functions and classes from the `PyMaterialXCore` and `PyMaterialXFormat`
+modules are available in the top-level `MaterialX` namespace.
+
+For example, the `PyMaterialXCore.Matrix44` class is available as
+`MaterialX.Matrix44`:
+
+.. code:: python
+
+    >>> import MaterialX as mx
+    >>> mx.Matrix44
+    <class 'MaterialX.PyMaterialXCore.Matrix44'>
+
+You can use the *Quick search* box in the sidebar on the left to quickly find
+documentation for a particular function or class of interest, for example
+`getUdimCoordinates <search.html?q=getUdimCoordinates>`_
+or `MeshPartition <search.html?q=MeshPartition>`_.
+
+
 MaterialX Python Modules
 ------------------------
 
@@ -46,3 +79,7 @@ MaterialX Python Modules
     PyMaterialXRenderGlsl
     PyMaterialXRenderOsl
     PyMaterialXRenderMsl
+
+.. toctree::
+
+    genindex