Use standard MaterialX nodes instead of MayaND nodes

**Is your feature request related to a problem? Please describe.**
Maya USD uses custom Maya MaterialX nodes that are not-portable unless other processes add the Maya node graphs into their search paths.
This means that Maya generated assets are unusable without a user understanding how to pipeline things, and cannot be shared without other users also having the same shader nodes available.

**Describe the solution you'd like**
Maya USD should stop using custom node defs. There should be an audit to see where they can use standard MaterialX nodes or embed the shading graphs into the output file.

**Describe alternatives you've considered**
Alternatively, Autodesk should contribute their nodes into the MaterialX standard.

**Additional context**
To reproduce:

1. Create a sphere with an OpenPBR shader in Maya 2026.
2. Attach a texture to the shader's color input
3. Export as a USDA with MaterialX selected in the Material export options
4. See that you now have nodes like MayaND_fileTexture_color4
5. Try and view this file in any other app that doesn't know about Maya's shader defs.

Reproduce the issue by creating a sphere with an OpenPBR shader in Maya 2026, exporting USDA with MaterialX enabled, and inspecting the MayaND_fileTexture_color4 nodes. Audit the exported graph for standard MaterialX alternatives or embedded shading graphs; done means the file can be viewed by an application without Maya shader definitions.

Written by the indexing model from the issue text.