* Start document consolidation.

* Transferred doc.

- Update checkerboard example with USD equiivalent and mermaid graph.
- Add USD/ / MaterialX / glTF mapping

…o spec README.

1. KHR_texture_procedural
2. EXT_texture_procedural_mx_1_39

…Add in mapping section for texture_infos.

Thanks for the fix.

… comes from the nodes.

Inputs on nodes can be aniated. Further discussion on defails on existing extension required.
Explicit note N:M graph -> material bindings possible.

* KHR_materials_ior: Minor typo: plural instead of singular (#2413)

* Update anisotropy extension spec (#2409)

* Clarify and simplify formula for attenuation. (#2414)

* Add TRACE prefix (#2417)

* Update clearcoat extension spec (#2415)

* Update README. (#2427)

* Add KHR_node_hoverability (#2428)

* Add KHR_node_hoverability
* Resort extension list

* EXT_mesh_gpu_instancing: Suggest extensionRequired usage (#2405)

* Add button/link to glTF Discord (#2429)

As there is a lot more traffic to the Khronos glTF Discord than there is to Slack, a button pointing users to Discord will be helpful.

---------

Co-authored-by: Andreas Atteneder <andreas.atteneder@unity3d.com>
Co-authored-by: Alexey Knyazev <3479527+lexaknyazev@users.noreply.github.com>
Co-authored-by: Ed Mackey <elm19087@gmail.com>
Co-authored-by: Marco Hutter <javagl@javagl.de>
Co-authored-by: Khronos Group Web Services <webservices@khronosgroup.org>
Co-authored-by: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>
Co-authored-by: James Riordon <outofcontrol@users.noreply.github.com>