Spec sheet with all mallard elements

We will install it to /usr/share/yelp/endless/spec/index.page, this
is not on the regular search path so it won't appear in any search
results.

However it can still be run from the command line as
yelp /usr/share/yelp/endless/spec/index.page
Allowing design to QA style changes.
[endlessm/eos-shell#4158]
Ditto