Add automated ML pipeline documentation generation

[gelluisaac]

Description

    Build automated documentation generation for ML pipelines including data flow diagrams, model cards, and pipeline metadata.

    ## Files to modify
    - astroml/pipeline/documentation/generator.py
    - astroml/pipeline/documentation/model_card.py
    - astroml/pipeline/documentation/data_flow.py
    - astroml/pipeline/documentation/templates/
    - astroml/api/routers/pipeline_docs.py

    ## 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. Implement pipeline metadata extraction
    2. Build model card generation (Google Model Cards spec)
    3. Add data flow diagram generation
    4. Implement pipeline documentation templates
    5. Create automated documentation API
    6. Add documentation versioning
    7. Implement documentation publishing
    8. Test with various pipeline configurations