Update user doc for datafusion integration.

[liurenjie1024]

Is your feature request related to a problem or challenge?

We made substaintial progress in datafusion integration, see #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

[xbattlax]

I've submitted a PR to address this: #2029

The documentation covers:

• Catalog-based access with IcebergCatalogProvider
• SQL operations (CREATE TABLE, INSERT INTO, SELECT)
• Metadata tables ($snapshots, $manifests)
• External tables via IcebergTableProviderFactory
• Partitioned tables and write modes (fanout vs clustered)
• Query optimization (projection, filter, LIMIT pushdown)
• Configuration options

Also added a working example at crates/examples/src/datafusion_integration.rs.