[Protocol] Add documentation for data skipping statistics configuration properties

[DrakeLin]

Description

This PR adds documentation to PROTOCOL.md for two table properties that control per-column statistics collection:

• delta.dataSkippingStatsColumns: Allows users to explicitly specify which columns should have statistics collected. Column names can refer to struct fields using dot notation (e.g., a.b.c), in which case all leaf fields within that struct are included.
• delta.dataSkippingNumIndexedCols: Specifies the number of leading leaf columns in the table schema for which to collect statistics. Defaults to 32.

This is implemented in Spark today. See

delta/spark/src/main/scala/org/apache/spark/sql/delta/DeltaConfig.scala

Line 637 in f86d549

val DATA_SKIPPING_NUM_INDEXED_COLS = buildConfig[Int](