Add epoch summary logging and update documentation

[sgreenbury]

This pull request improves the visibility of training timing information by adding a human-readable summary of epoch timings to the logs at the end of training. This allows users to quickly inspect timing quality without needing to load the checkpoint file.

Logging and user experience improvements:

• Added a summary log at the end of training that reports the number of epochs, mean, minimum, and maximum epoch durations, making it easier to assess training timing directly from logs. For short runs (≤12 epochs), the per-epoch times are also listed. (src/autocast/scripts/training.py)
• Updated documentation to describe the new timing summary log output, clarifying that users can inspect timing quality without loading the checkpoint. (docs/SCRIPTS_AND_CONFIGS.md)