add paper references, missing docstrings, and YAML comments

[pattyboy227]

• Expand module-level docstrings in dataset, model, and task modules with Boag et al. 2018 paper citation and URL
• Add missing docstrings to private helper functions and methods: _path_variants, _table_assets_exist, _discover_optional_tables, _infer_tensor_input_size, _mean_pool_sequence, _project_tensor, _embed_text_field, _require_columns, _coerce_timestamp, _normalize_token, _normalize_code_status_mode, _normalize_dataset_prepare_mode, _calculate_age_years, _calculate_los_days, _calculate_paper_like_los_days, _build_code_status_target_normal, _build_code_status_target_paper_like
• Add section comments to eol_mistrust.yaml config for table groups
• Enhance RST documentation with paper links and modality descriptions
• Add paper reference to example script header
• Add docstrings to four public wrapper functions in pyhealth/models/eol_mistrust.py that bind the generic proxy helpers to specific label columns