[REQ] Add JSON schema for openapitools.json

Hello dear colleagues!

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

I'm having difficulty configuring the openapitools.json file. There is no JSON-schema described for the file and there is no official way, except for examples from StackOverflow, to write a valid configuration.

Describe the solution you'd like

Please add the openapitools.jsonJSON-schema to the docs/ folder.

Describe alternatives you've considered

Alternative solution is describe a link to JSON-schema in the documentation or generators files. For example:
"$schema": "https://host.com/openapitools-schema.json",

Additional context

This enhancement was requested by me because of the need to understand the source code of the project for a long time, since I needed to change the target folder for the generated TypeScript models. Explicitly setting the folder property did not help. Later it turned out that this was impossible

For more additional context :) you can read my another enhancement related to openapitools.json https://github.com/OpenAPITools/openapi-generator/issues/18057

Start in the docs/ folder and review the source code and generator files that define openapitools.json configuration. Identify the supported fields and produce a JSON schema that covers them, then verify it against the available configuration examples and document or link its use.