Organize API docs and adjust language stats

[mhmdaskari]

Summary

• Splits the generated API reference into a nested MkDocs tree for configuration, data, preprocessing, BCI IV-2a conversion, GAN augmentation, framework models, plotting, CLI, and constants.
• Adds a .gitattributes Linguist override so .ipynb files stay in the repository but do not count toward GitHub's language statistics.

Validation

• CUDA_VISIBLE_DEVICES=-1 mkdocs build --strict
• git check-attr linguist-detectable -- notebooks/00_data_postprocessing.ipynb