Add automated data validation with Great Expectations

[gelluisaac]

Description

    Integrate Great Expectations for automated data validation, expectation suites, and data documentation.

    ## Files to modify
    - astroml/validation/great_expectations/suite_builder.py
    - astroml/validation/great_expectations/validator.py
    - astroml/validation/great_expectations/data_docs.py
    - astroml/api/routers/data_validation.py
    - configs/validation/ge_config.yaml

    ## Out of scope
    - Refactoring unrelated modules
    - Changing public API contracts
    - Major version bumps or breaking changes
    - Performance optimizations not directly related to the issue

    ## Acceptance criteria
    - [ ] Code changes implemented as per procedure steps
    - [ ] All new code has complete type hints
    - [ ] Unit tests added/updated with >90% coverage for changed code
    - [ ] All tests pass: pytest tests/
    - [ ] Linting passes: pre-commit run --all-files
    - [ ] Documentation updated if needed
    - [ ] Changes reviewed and approved

    ## Procedure
    1. Set up Great Expectations configuration
    2. Implement automated expectation suite generation
    3. Build data validation pipeline integration
    4. Add data documentation generation
    5. Implement validation results storage
    6. Create validation dashboard
    7. Add CI/CD integration for data validation
    8. Test with various data sources