Design System: Implement colors page

## Description

We are migrating our design system to Storybook. This issue is about creating a page for colors. Compared with our previous style guide https://graylog2.github.io/frontend-documentation/#section-colors it should not just list all the available colors, it should only list the relevant color and give you an idea of "when should I use which color?". It should not list the deprecated colors (e.g. `theme.variant` or `theme.gray`).