[DOC] Remove all-contributors

[HugoDelatte]

Reference Issues/PRs

What does this implement/fix? Explain your changes.

all-contributors doesn't support rst file. We will default to the Github contributors display until all-contributors fix this issue

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

Any other comments?

PR checklist

For all contributions

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• I've added myself to the list of contributors
  How to: add yourself to the all-contributors file in the skfolio root directory (not the CONTRIBUTORS.md).
• Optionally, I've added myself and possibly others to the CODEOWNERS file - do this if you want to become the owner or maintainer of an estimator you added.

For new estimators

• I've added the estimator to the API reference in docs/api.rst.
• I've added one or more illustrative usage examples to the docstring and the examples section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (18073ad) 99.15% compared to head (7fd7a48) 99.15%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   99.15%   99.15%           
=======================================
  Files          35       35           
  Lines        2593     2593           
=======================================
  Hits         2571     2571           
  Misses         22       22           

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