docs: model_selection module docstrings

[mkalimeri]

Description

Related to #596

Examples added for

• model_selection.TimeGapSplit
• model_selection.GroupTimeSeriesSplit
• model_selection.ClusterFoldValidation (I see that KlusterFoldValidation has been renamed as ClusterFoldValidation)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines (ruff)
• I have commented my code, particularly in hard-to-understand areas
• [NA] I have made corresponding changes to the documentation (also to the readme.md)
• [NA] I have added tests that prove my fix is effective or that my feature works
• [NA] I have added tests to check whether the new feature adheres to the sklearn convention
• New and existing unit tests pass locally with my changes

If you feel your PR is ready for a review, ping @FBruzzesi or @koaning.

[FBruzzesi]

Thanks for the PR @mkalimeri!

The only concern I have is related to the part of code in TimeGapSplit summary. @koaning WDYT?
We can keep it as a follow up, but that's certainly not the right assumption to make in this context

[mkalimeri]

I updated the code based on the feedback from the previous review. I kept the summary field 'num of days' and updated the calculation to reflect the actual number of days. I also added a field 'frequency', to inform about the frequency of the data. I updated example/test to reflect these changes. Let me know if you can think of other informative fields!