Define the GitHub workflows

It looks like https://github.com/KhronosGroup/glTF-Community/blob/main/CONTRIBUTING.md contains an overview of the contribution process, and https://github.com/KhronosGroup/glTF-Community/blob/main/vendors/README.md contains additional details and a checklist (which we might make part of some PR template...?). But it looks like there's still some infrastructure missing for properly integrating these contributions. For example, I assume that files like https://github.com/KhronosGroup/glTF-Community/blob/main/extensions/README.md should be built automatically as part of the CI. We should clarify which of the files of the repo are processed and auto-generated, to prevent people from manually tweaking files that are then going to be overwritten anyhow.

Start by reading CONTRIBUTING.md, vendors/README.md, and extensions/README.md to understand the contribution process and the files that may need integration. Inventory which repository files are generated or processed, then define the required GitHub workflows and document what is automated so contributors know what not to edit manually.