feat(VSelectionControl): add text prop for description below label

[sridhar-3009]

Summary

Fixes #21773 — adds a text prop to v-checkbox (and all other selection controls) that renders a description below the checkbox label.

Changes

• VSelectionControl.tsx: add text: String prop and text slot; wrap label + text in .v-selection-control__content flex-column container
• VSelectionControl.sass: add styles for .v-selection-control__content and .v-selection-control__text
• _variables.scss: add $selection-control-text-font-size (0.875rem) and $selection-control-text-color SASS variables
• Docs: add prop-text example and section for v-checkbox

Usage

<v-checkbox
  label="Subscribe to newsletter"
  text="You'll receive weekly updates about new features and releases."
/>

The text slot can be used for rich content:

<v-checkbox label="Accept terms">
  <template #text>
    I agree to the <a href="/terms">Terms of Service</a>.
  </template>
</v-checkbox>

Test plan

• Render a v-checkbox with the text prop — description should appear below the label
• Verify font size is smaller than the label but larger than hint text
• Verify the text slot override works
• Check that v-radio and v-switch also accept the prop (as they share VSelectionControl)
• Verify no visual regression when text is not set

[J-Sek]

• rebase to dev
• provide Playground snippet in PR description (something similar to quick demo + some use cases where text wraps 2 or more lines)
• v-switch requires additional padding