Make theme desktop responsive breakpoint configurable

### 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).

### Motivation

See
- https://github.com/facebook/docusaurus/pull/9261
- https://github.com/facebook/docusaurus/pull/9335

People want to be able to configure easily the 996px desktop breakpoint.

It's not easy currently because the value is hardcoded in both the JS side, but also CSS side (inside Infima directly), cf my comment https://github.com/facebook/docusaurus/pull/9261#issuecomment-1700876844

We should make this easier to achieve without requiring swizzling.

### Self-service

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