Bug 961146 - Refactor Health Report documentation, import external docs; r=rnewman

FHR docs were extracted from /services/docs into their own compartment.
This arguably should have been done originally.

Documentation on the payload format has been imported from
https://github.com/mozilla-services/docs. Those docs were created in a
time before we had in-tree docs and this tree is a more logical home for
them.

--HG--
rename : services/docs/healthreport.rst => services/healthreport/docs/index.rst
extra : rebase_source : 4e2fd62cc98ea2620f580e34d032cb39bafb5108
extra : amend_source : bd6309cc9d53aba1b085067960bf20a94125c5b0