[Slider] Add usage docs for slider with custom label

[kingtraceyj]

Acceptance criteria

Update the Slider Usage tab docs to include guidance on having a custom value label. It might be a good idea to put this guidance under the modifiers section as a starting point.

Tasks

• [ ] Document when do labels change from one to another.
• [ ] Document the intended responsiveness of the slider component.

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Ref PR: https://github.com/carbon-design-system/carbon/pull/16616

[laurenmrice]

• [ ] @laurenmrice Follow-up with Tracey with what she meant by:

  how many tick marks are allowed?

The addition of tick marks might make more sense for the design in the incremental slider proposal. This enhancement was demonstrating you can just use words instead of numbers for the label of our current functionality.