Write docusaurus.config.js with an interactive UI

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Motivation

Writing the docusaurus.config.js file is very tedious, especially if it gets long. The theme config object bloats very easily, and it's hard to navigate between different fields.

We should allow the debug plugin to write to docusaurus.config.js. Not everything is serializable, but we can allow users to write JS code with a text area; many other fields only accept certain values / boolean values, and they can be easily implemented with form elements.

### Self-service

- [X] I'd be willing to do some initial work on this proposal myself.