docs: removed redundant files/code and added docstrings for artifacts/ (#175)

## Summary

- Deleted 15 dead files:
  - entire DataChecksProcessor pipeline
    - data_checks_processor.py
    - base/manifest.py
    - base/metadata.py
    - base/metrics.py
    - base/analyzer.py
    - base/name_anonymizer.py
    - base/data_checks.py
  - Entire analyzers/data_checks/ directory (8 files): 
    - __init__.py
    -  base.py
    - dataset_size.py
    - high_float_precision.py
    - missing_data.py
    - sparse_data.py
    - surrounding_whitespaces.py
    - text_data.py
- Partial cleanup in 3 files:
- `analyzers/field_features.py`: Removed `FieldFeaturesAnalyzer` class,
5 dead helper functions (`_add_info_to_manifest`, `_add_ner_to_fields`,
`_type_summary`, `_calculate_aggregates`, `_max_aggregate`), and 8 dead
imports.
- `base/fields.py`: Removed `FieldFeaturesInfo` class and unused
`cached_property` import.
  - `base/__init__.py`: Removed unused `ArtifactProcessingException`.
- Added docstrings to all 5 remaining files:


Ensure that the following pass:

- [x] `make format && make check` or via prek validation.
- [x] `make test` passes locally
- [x] `make test-e2e` passes locally
- [ ] `make test-ci-container` passes locally (recommended)

## Other Notes

made by Cursor

Signed-off-by: Sean Yang <seayang@nvidia.com>

Author: seayang-nv