Add navigation tree component(s)

[james-nash]

A number of projects require UI component(s) to display and navigate potentially long and deep trees of links:

• Architecture site, design systems glossary site and Gravity hub site will probably need something like this for their navigation. Think of the left-hand navbars you tend to see in styleguides (e.g. Storybook or Fractal), wikis and glossary sites
  • Our existing nav links component - which is intended for the top-level site navigation isn't really designed for very large sets of links
• Table of contents jump links at the top of a long article or post