Add dask to __init__.py and fix its documentation generation.

[y0z]

Motivation & Description of the changes

• Add the dask integration to __init__.py.
• Generate the documentation for the dask integration from optuna_integration.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (2734577) 58.03% compared to head (e9bc721) 57.99%.

Files	Patch %	Lines
optuna_integration/__init__.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================== - Coverage 58.03% 57.99% -0.04% ========================================== Files 24 24 Lines 1506 1507 +1 ========================================== Hits 874 874 - Misses 632 633 +1 ```

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