Support the parsing of Apple's Keynotes

### Requested feature

This issue consists of enabling Docling with parsing capabilities for Apple's iWork presentation files (**Keynotes**).

- This feature should be aligned with the existing parsers of Apple's iWork documents (**Pages**), implemented in `IWorkPagesDocumentBackend`. Note that there is an open issue to enhance its capabilities (#4025 ), as well as an issue to support iWork spreadsheets (**Numbers**).
- It should try to match the capabilities of existing backend parsers for presentations: `MsPowerpointDocumentBackend` (Office PowerPoint) and `OdsDocumentBackend` (OpenDocument Spreadsheet)
- The documentation should be updated to reflect this new feature: in README (_What's new_), [index.md](https://github.com/docling-project/docling/blob/main/docs/index.md), and [supported_formats.md](https://github.com/docling-project/docling/blob/main/docs/usage/supported_formats.md)

Just drop a line if you're interested in resolving this issue. Make sure to follow the [CONTRIBUTING.md](https://github.com/docling-project/docling/blob/main/CONTRIBUTING.md). If leveraging AI assistants, ensure that the code standards described in [AGENTS.md](https://github.com/docling-project/docling/blob/main/AGENTS.md) are enabled.

### Alternatives

This is a new feature are the are currently no alternatives in Docling