docs: Update Parquet scan documentation

[andygrove]

Overview

This PR removes all references to the deprecated native_comet scan implementation from the documentation and
configuration, and improves the accuracy and clarity of the Parquet scan documentation.

Changed Files

common/src/main/scala/org/apache/comet/CometConf.scala

• Changed the category of spark.comet.scan.impl from CATEGORY_SCAN to CATEGORY_PARQUET
• Rewrote the doc string to describe native_datafusion and native_iceberg_compat without referencing
  native_comet
• Removed the .internal() marker, making this configuration visible to users

docs/source/contributor-guide/parquet_scans.md

Major rewrite of the Parquet scan documentation:

• Removed all references to the deprecated native_comet scan (previously listed as one of three implementations)
• Removed the comparison table that included native_comet and the "benefits over native_comet" section
• Removed the separate native_comet S3 section (which described Hadoop-AWS-based S3 access)
• Updated the S3 configuration and examples sections to reference both native_datafusion and native_iceberg_compat
  (previously only referenced native_datafusion)
• Clarified that auto mode currently always selects native_iceberg_compat
• Separated limitations into two clear categories:
  • Fallback to Spark (safe): unsupported features that cause Comet to fall back to Spark, producing correct
    results with reduced performance
  • Potential incorrect results: issues that do not fall back and may produce wrong answers (datetime rebasing
    for both scans, hard-coded config defaults for native_iceberg_compat)
• Added previously undocumented native_datafusion limitations that cause fallback:
  • Dynamic Partition Pruning (DPP)
  • input_file_name(), input_file_block_start(), input_file_block_length() SQL functions
  • Spark metadata columns (e.g., _metadata.file_path)
• Added Parquet encryption as a shared fallback limitation
• Fixed misleading wording for ignoreMissingFiles/ignoreCorruptFiles (previously said "not compatible with Spark",
  now clarifies it falls back to Spark)
• Removed stale issue links (#1545, #1758) that referenced old native_datafusion issues

docs/source/contributor-guide/ffi.md

• Replaced reference to native_comet with a general description of scans that use mutable buffers

docs/source/contributor-guide/roadmap.md

• Removed the "Removing the native_comet scan implementation" roadmap section (now completed)
• Simplified the Iceberg integration description by removing the mention of the native_comet to
  native_iceberg_compat transition