LHS navigation for Docs

Created by: ryan-blunden

Tasks

@ryan-blunden Hard-coded nav HTML
@francisschmaltz Implement nav style and functionality
- Nav stays fixed so scrolling the content doesn’t scroll the nav.

Currently, users must rely on inter page links to navigate and discover different parts of our documentation.

Adding a hierarchical representation of the site’s structure would let users browse to see if the info they're looking for matches a section or page.

Example from readme.io that has nice functionality.