[ENHANCEMENT] argilla: align usage of questions

[frascuchon]

Description

This PR reviews and aligns the questions' usage with the rest of the settings. Now, questions can be accessed as the rest of the settings:

settings.questions["name"]

Note: Updating questions for existing datasets is still not possible. But we can add questions to settings before creating the dataset:

settings.questions.add(...)
dataset = rg.Dataset(..., settings=settings).create()

Closes #5231

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)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[davidberenstein1957]

LFTM, perhaps we should double check the docs?

[frascuchon]

LFTM, perhaps we should double check the docs?

@nataliaElv is already working on this here