Export public APIs in top-level package

[tiran]

Fixes #63 and #30

8f014b0 Export public APIs in top-level package
35fe7ee Export more public APIs in top-level package
40b45ad Export more public APIs in top-level package

commit 8f014b0a6960aab78d6b3879eed8d61ba5ab7e16

    Export public APIs in top-level package
    
    Export `SDG`, `GenerateException`, and `generate_data()` in the
    top-level package name. Add notes to `generate_data` and
    `GenerateException` that the function and class are part of the public
    API.
    

commit 35fe7ee65b6e0e2db1ce2fb7d0a4bb1c7f6ba280

    Export more public APIs in top-level package
    
    Adding to `instructlab.sdg`:
    
    - `Block`
    - `EmptyDatasetError`
    - `FilterByValueBlockError`
    - `Pipeline`
    - `PipelineConfigParserError`
    - `PipelineContext`
    - `SIMPLE_PIPELINES_PACKAGE`
    - `FULL_PIPELINES_PACKAGE`    

commit 40b45ad8196087fda02887a9257769d285c7302a
    Export more public APIs in top-level package
    
    Adding to `instructlab.sdg`:
    
    - `CombineColumnsBlock`
    - `ConditionalLLMBlock`
    - `FilterByValueBlock`
    - `ImportBlock`
    - `LLMBlock`
    - `SamplePopulatorBlock`
    - `SelectorBlock`

[markmc]

See #63

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. @tiran please rebase it. https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[russellb]

I should have rebased this before merging. It looks like it broke CI. I'll submit a follow-up fix for that.