Update CONTRIBUTING.md file with new UI components.
Update CONTRIBUTING.md file with new UI components.

- Remove instructions for outdated components
- Add instructions for new components (note, warning and panels)
- Add general UI advice

yvonnewilson/docs,yvonnewilson/docs,jeffreylees/docs,auth0/docs,auth0/docs,jeffreylees/docs,yvonnewilson/docs,jeffreylees/docs,auth0/docs