[TO_REVIEW] Add utilities functions to the doc

[antoinecollas]

Updated Documentation:

• skada.source_target_split
• skada.per_domain_split
• skada.utils.check_X_y_domain
• skada.utils.extract_source_indices
• skada.utils.extract_domains_indices
• skada.utils.source_target_merge

Issue Resolved:
The example for skada.utils.source_target_merge failed when y_target=None because the function was generating y_target with _DEFAULT_MASKED_TARGET_REGRESSION_LABEL, even for classification tasks. This caused a type mismatch when merging arrays (int vs. nan). nan were then casted as int with specific value -9xxxxx.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.07%. Comparing base (3647fd9) to head (133f0dd).
Report is 1 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (3647fd9) and HEAD (133f0dd). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (3647fd9)	HEAD (133f0dd)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #227       +/-   ##
===========================================
- Coverage   97.24%   83.07%   -14.17%     
===========================================
  Files          54       46        -8     
  Lines        5620     5259      -361     
===========================================
- Hits         5465     4369     -1096     
- Misses        155      890      +735