Improve typography design documentation

Our current [Design/Typography page](https://v-next--cloudfour-patterns.netlify.com/?path=/docs/design-typography--headings) mentions that it is a work in progress. This page would probably benefit from some additional information:

- [ ] Reasoning behind Source Sans usage: Open source, readable, humanist, somewhat academic in appearance.
- [ ] Variable font demonstration. Maybe a weight slider like [in @dromo77's article](https://cloudfour.com/thinks/get-more-for-less-with-variable-fonts/)?