Metrics definition as JSON

[mweidling]

Describe the feature you'd like
In order to create tool tips for the metrics in the Quiver UI we need the metrics in a parsable fashion somewhere.
A first idea is to provide a JSON file that includes the short descriptions of the metrics from the JSON schema (see OCR-D/spec#225) as well as the link to the full definitions (still to be merged). This file could then be fetched and parsed by the front end.

User story
As a user of the benchmark overview I need some explanation of the benchmarking metrics in order to understand what all the numbers in the UI mean.

Additional context

• QA Spec spec#225 for the current schema and full definitions