Memoize the result of PublishingAPIManual.to_h

We're calling it multiple times, but it should always be the same, so let's
cache it.
Push sections to Rummager

Like manuals themselves, this functionality will move to a component downstream
of Content Store, but for now we need to implement it here.
Memoize PublishingAPISection#to_h

We're calling it multiple times and it does some non-trivial work.