docs: expand tuning guide with performance and memory configs

[andygrove]

Summary

Audited all CometConf entries in preparation for the 1.0 release and expanded the Tuning Guide with the user-facing performance and memory knobs that were previously undocumented.

New sections in tuning.md:

• Batch Size — spark.comet.batchSize (memory vs. throughput tradeoff) and its shuffle-writer cousin spark.comet.columnar.shuffle.batch.size
• Limiting Spill Disk Usage — spark.comet.maxTempDirectorySize (100 GB default)
• Parquet Reader Tuning — parallel I/O thread pool, range coalescing, filter pushdown / late materialization
• Iceberg Scan Tuning — spark.comet.scan.icebergNative.dataFileConcurrencyLimit
• Reducing Row/Columnar Conversion Overhead — spark.comet.exec.transitionRevert.{enabled,maxTransitions}
• Metrics Overhead — why spark.comet.metrics.enabled is off by default
• Expanded Shuffle Compression with codec / zstd level knobs

Also:

• Fixed an incorrect claim that Comet overrides Spark LZ4 shuffle compression with ZSTD (the default codec is lz4).
• Moved spark.comet.maxTempDirectorySize from the exec category to tuning so it renders in the Memory & Tuning table in configs.md, and expanded its doc string.

Test plan

• Docs build renders new sections correctly
• configs.md still generates cleanly (moved config appears in Memory & Tuning table)