[DOC] Code use of "estimator" term in base object interfaces

[tpvasconcelos]

Reference Issues/PRs

Depends on https://github.com/sktime/skbase/pull/281

What does this implement/fix? Explain your changes.

See https://github.com/sktime/skbase/pull/281#issuecomment-1956646894 and relevant replies.

What should a reviewer concentrate their feedback on?

PR checklist

For all contributions

• [x] I've reviewed the project documentation on contributing
• [x] I've added myself to the list of contributors.
• [x] The PR title starts with either [ENH], [CI/CD], [MNT], [DOC], or [BUG] indicating whether the PR topic is related to enhancement, CI/CD, maintenance, documentation, or a bug.

For code contributions

• [x] Unit tests have been added covering code functionality
• [x] Appropriate docstrings have been added (see documentation standards)
• [x] New public functionality has been added to the API Reference