Add current page title to top of nav for docs pages

[justmoon]

Right now it's impossible to tell what document you're reading unless you scroll to the top:

On mobile, the menu is placed above the content:

So again it's hard to tell what page you're on.

Both could be fixed by adding the page title to the top of the nav.

[mynetx]

@justmoon Define "nav" in this context – do you want to add it to the dark bar, or to the side/top in-page menu?