'usage.md' documentation provides incorrect parameters for sources

The documentation `docs/usage.md` indicates I should pass a url to docling-serve like:

```json
{
"http_sources": [{"url": "https://arxiv.org/pdf/2206.01062"}]
}
```

However, with recent docling-serve images (I'm running 1.9.0), this appears to be incorrect, and instead I should use:

```json
{
"sources": [{"kind": "http", "url": "https://arxiv.org/pdf/2206.01062"}]
}
```

I suggest updating the documentation to reflect the current API, and ensure that all available source types are represented (it appears that now `s3` is also/newly available?)

Start by reading docs/usage.md and compare its http_sources example with the current docling-serve API described in the issue. Confirm the current source syntax and identify every available source type, including whether s3 is supported. Done means the documentation uses the current sources format and represents all available source types accurately.

Written by the indexing model from the issue text.