[Feature Request] Add official json schema definition for docfx.json and toc.json

[filzrev]

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)

[KalleOlaviNiemitalo]

How can a JSON schema be generated from C#, for a property whose value can be a string or an object? Like in the file mapping settings.

[filzrev]

There are limitations on automatically generated JSON schemas.
And JSON schema generation from C# code is not recommended for production usage.
https://docs.json-everything.net/schema/schemagen/schema-generation/#schema-schemagen-disclaimer

So I've decided to manually write JSON schema based on generated schema.

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Currently It Visual Studio don't supports JSON Schema 2019-09 or later.
It need to write JSON Schema with Draft-07 version for auto completion.

[KalleOlaviNiemitalo]

There's schema-generation work at dotnet/aspire#3309; but that looks specific to Microsoft.Extensions.Configuration.Binder and wouldn't recognize the JSON-specific attributes that DocFX uses in its configuration classes.