[core] feat(FormGroup): add support for "sub label" text

[mud]

Fixes #5019

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

To resolve #5019 this PR adds a new prop labelHelperText to FormGroup which adds helperText between label and children.

Reviewers should focus on:

We could have ignored the labelHelperText if label is empty, but decided against it as it seemed to add unnecessary complexity. Similarly, there's no restriction on specifying only one type of helperText. It looks slightly odd when both helper texts are used, but we may end up using both with one of our use case where we have a list of fields specified as children.

Screenshot

[blueprint-bot]

Remove unused class

Previews: documentation | landing | table | modern colors demo