Make Onionoo's document classes available as part of metrics-lib
This ticket is based on a discussion on legacy/trac#24036 (moved):
- At some point in the future we may want to move parts of
org.torproject.onionoo.docs
to metrics-lib, so that metrics-bot and other clients can re-use that code. More specifically, we should only move the externally provided JSON objects, not the internally used status objects.