Update user doc for datafusion integration.

### Is your feature request related to a problem or challenge?

We made substaintial progress in datafusion integration, see https://github.com/apache/iceberg-rust/issues/1382. I think it's a good time to update user doc so that more user knows how to use it.

### Describe the solution you'd like

_No response_

### Willingness to contribute

None

Start by reading the linked datafusion integration issue, #1382, and locate the existing user documentation for integration-related usage. Determine which supported workflows need explanation and update the relevant documentation so users can understand how to use the integration; confirm that the documented steps match the current implementation.

Written by the indexing model from the issue text.