Improve TypeScript Types for ThemeConfig

### 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#issues).

### Description

I would like to suggest providing better TypeScript types for `ThemeConfig` to improve the developer experience.

### Has this been requested on Canny?

_No response_

### Motivation

Currently, the `ThemeConfig` type is defined as:

```
export type ThemeConfig = {
[key: string]: unknown;
};
```

As a result, developers are unable to benefit from IDE IntelliSense when configuring `ThemeConfig` within `docusaurus.config.ts`.

Improving the type definition would greatly enhance the DX by enabling better autocompletion, error checking, and documentation directly within the editor.

### API design

_No response_

### Have you tried building it?

_No response_

### Self-service

- [ ] I'd be willing to contribute this feature to Docusaurus myself.