Docs: expand seed dataset docs for filesystem seed sources and fix preview example typo

[stepwise-ai-dev]

Summary

The seed dataset docs currently lag the shipped API and contain a small example bug.

Problems

• docs/concepts/seed-datasets.md documents only three seed source types, but the code and tests now support five.
• The page does not explain DirectorySeedSource or FileContentsSeedSource.
• The complete example uses designer.preview(...) instead of data_designer.preview(...).

Proposed fix

• Update docs/concepts/seed-datasets.md
• Add sections for DirectorySeedSource and FileContentsSeedSource
• Document file_pattern, recursive, encoding, and exposed seeded columns
• Fix the preview example typo
• Add a brief note pointing advanced users to custom filesystem seed readers/plugins

Source of truth

• packages/data-designer-config/src/data_designer/config/seed_source.py
• packages/data-designer-engine/src/data_designer/engine/resources/seed_reader.py
• packages/data-designer/tests/interface/test_data_designer.py
• packages/data-designer-engine/tests/engine/resources/test_seed_reader.py