fix: slider vertical navigation and value setting bugs

[marjonlynch]

Description

• fixed issue with arrow keys navigating the whole page as well as the silder
• fixed bug where initial value of slider was not setting the thumb position correctly
• fixed bug in vertical orientation where movement via mouse was not correct
• Added headers to all the examples in slider and radio-group
• Fixed color for radio-group slotted labels in fast-components

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.