Better document valid/invalid json manifests

[charles-lunarg]

**What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. **
Recently, an issue was reported that came from the Shader Object Extension Implementation Layer using a raw number instead of a string in its manifest. The loader expects a string, and the examples in the docs use strings. Yet this isn't obvious to the reader since it isn't clearly defined.

There are two solutions, one is to document the exact json types the loader expects in every field, and the second is to create a proper JSON Schema that can be used programatically.