Consider changing doc URL structure

[gregjd]

Switch to have docs nested under places, i.e. opendatapolicies.org/doc/seattle-wa/2016-02-01/ instead of opendatapolicies.org/doc/seattle-wa-2016-02-01/, since every doc is associated with exactly one place. This will reduce the redundancy between doc titles and doc metadata. Could pair this with making place pages visible. (But maybe, then, both the place and doc URLs should be /docs/.)