Drop _load and _save in core datasets and docs

[deepyaman]

Description

Take advantage of the ability to define load and save publicly in Kedro core (separate PR for Kedro-Datasets upcoming), and provide guidance to users to do the same.

There's no new core functionality introduced here.

Development notes

I removed this from the initial #3920, due to potentially breaking changes for Kedro-MLflow users. Now, Galileo-Galilei/kedro-mlflow#590 has been merged (I had created this PR months ago, as you can see, but had been slow on unblocking Kedro-MLflow), so there is a clear path forward for Kedro-MLflow.

Developer Certificate of Origin

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team