fixing mobile navigation, and adding composing page to nav

This is in reference to issues #75 (closed) and #74 (closed) - I took the liberty of editing the extra.css file directly, to avoid forking & submitting a PR to the external antora-ui-default fork by @robinbb.

The resulting navigation looks decent - it's a static, 100% wide version at the top of each page - this lets users scroll past it with ease, and it should be "where users expect it to be".