Sidenav builder doesn't handle external URLs

[sanscontext]

See: #282 (review)

Expected: When I include a fully-qualified domain name as a path in the nav, it should just make that item in the nav a link to the external site

Actual: When I click the link, it prepends the docs root to it like so: https://app.segment.build/docshttps:/reference.segmentapis.com/

[XavierAgostini]

fixed this