Update design templates to match standard website

[practicalparticipation]

The design of the standard documentation should reflect the design at http://standard.open-contracting.org/

We should consider whether it should also share the same top-level navigation, so that, from a user perspective, clicking 'standard' on either site takes you to http://ocds.open-contracting.org/standard/r/{latest}, whereas clicking 'project' (etc.) would take you to the pages at http://standard.open-contracting.org/

[practicalparticipation]

Rough draft of site template:

[practicalparticipation]

Note how on the standard site the menu bar remains on screen when scrolling.

[foobacca]

Done the header, but for now using the exact same set of links from the the standard site. But it's easy to add/remove links