[BUG] Documentation for online hosted generator is missing

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The documentation for the online hosted generator here:
https://github.com/OpenAPITools/openapi-generator/blob/master/docs/online.md
simply provides a link to the hosted generator web page. There is no documentation on how to use it.

Also, there is a button "LOCAL JSON FILE" but it doesn't appear to do anything.

openapi-generator version

5.2.1 http://api.openapi-generator.tech/index.html

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Go to http://api.openapi-generator.tech/index.html and try to generate a client from an uploaded JSON file, without any instructions

Related issues/PRs

Suggest a fix

Take some time to complete the codumentation on how to use the online hosted tool.

Start by reading docs/online.md and opening the hosted generator at api.openapi-generator.tech/index.html. Document the steps for generating a client from an uploaded JSON file, including the purpose and behavior of the “LOCAL JSON FILE” button. The work is done when a newcomer can use the hosted tool without needing to infer the workflow.