Document color system in contribution guide

[robyngit]

@yvonnesjy is working on coming up with a color management system for portals, which will simplify styling new components and creating new portal themes. We should adopt these concepts to MetacatUI as a whole. This task entails documenting the color system principles clearly in the CONTRIBUTING doc and indicating there that we will apply them to at least new/modified code moving forward.

References:

• https://github.com/NCEAS/metacatui/pull/2436
• WIP doc: https://docs.google.com/document/d/1Eb0uixayVeEgO0AcIEjDO6J5555A_CA1BK0nZp1s-uk/edit