[docs] add vaex-ml to list of external repositories

[jameslamb]

vaex is a popular Python DataFrame library, offering the ability to work with larger-than-memory datasets on disk (similar to Dask DataFrame or sqlite).

Noticed based on @ddelange's commits referencing #5196 that vaex ships an integration to LightGBM, as part of a package called vaex-ml.

https://github.com/vaexio/vaex/blob/15245cf4332d4423ac58bd737aee27d911a1b252/packages/vaex-ml/vaex/ml/lightgbm.py#L21

This proposes adding that project to the list of external repositories in the documentation here.

cc @JovanVeljanoski , the original author of this from vaexio/vaex#254

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.