chore(examples): Use QuantileDMatrix for histogram tree method in XGBoost example

[sunalawa]

Replace DMatrix with QuantileDMatrix in distributed XGBoost training example.
This reduces memory usage and aligns with XGBoost best practices for distributed workloads

What this PR does / why we need it:

Updates the distributed XGBoost example to use QuantileDMatrix instead of DMatrix.
This reduces memory usage and follows XGBoost best practices for distributed training workloads.

Which issue(s) this PR fixes

Fixes #3300

Checklist:

• Docs included if any changes are user facing

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Pull request overview

Updates the distributed XGBoost training example notebook to use QuantileDMatrix instead of DMatrix, aiming to reduce memory usage and align the example with histogram-based training best practices.

Changes:

• Replace xgb.DMatrix with xgb.QuantileDMatrix for training and validation data construction.
• Update the notebook note to reflect QuantileDMatrix construction requirements within the communicator context.