docs : sync 6 doc files to actual code
- architecture.md: removed TrainConfig.log_interval, split KVCache into
PageCache/ContiguousCache with CacheView/PageCacheView/ContiguousCacheView,
added JsonlStore, fixed GradientCheckpointingCallback type,
CheckpointCallback typo, ProgressBarCallback hooks
- training.md: added position_ids to SFT keys, fixed callback hook table,
removed merged ValidationCallback
- inference.md: documented ContiguousCache default vs PageCache paged
- dataflow.md: added JsonlStore to storage backends and format detection
- params.md: removed nonexistent --log_interval
- preprocessing.md: updated timestamp
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