classNames util documentation and using it in example project

[szilvihorvath]

Issue: https://github.com/Capgemini/dcx-react-library/issues/412

• documentation added to Storybook
• className util added to index.ts file to be packaged
• using className util from dcx-react-library in the example project

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (a2cd406) compared to base (c570b6c). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/0.7 #416 +/- ## ============================================= Coverage 100.00% 100.00% ============================================= Files 45 45 Lines 1029 1029 Branches 400 400 ============================================= Hits 1029 1029 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Capgemini). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Capgemini)

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