Add descendant focus/active example

[leyanlo]

The Styletron documentation has a section on Descendant Hover. In practice, we typically include hover styles with focus/active styles for accessibility. Therefore, we should include documentation on how to add descendant focus/active styles.

[gergelyke]

@leyanlo would you mind creating a PR adding a section to https://github.com/styletron/styletron.org/blob/master/pages/concepts.mdx?