Is your feature request related to a problem? Please describe.

When editing docfx.json and toc.yml.
It'll be helpful intellisense are supported by default. (It require json schema definition)

Describe the solution you'd like

Add json schema definition files to docfx repository's following directory.

https://github.com/dotnet/docfx/tree/main/schemas
├ docfx.json
└ toc.json

These json schemas should be generated from C# code to minimize maintenance cost.
(with tool like NJsonSchema or JsonSchema.Net.Generation)
and should be tested by CI automatically.

Additional context
There is unofficial schema definition for docfx.json exists.
and schema for toc.yml is discussed at #7736 (expanded and dropdown options are added for toc.json recently)