Improve documentation of KPIs

## Description

GERBIL already offers several different measures. Especially the different "variations" of precision, recall and F1-measure can lead to confusions (e.g., #320). Although we have documented the measures, the "accessibility" of the explanation could be increased.

## Solution

* Use RDF metadata for the measures
* most measures already have a URI that is used for the generation of RDF data
* the meta data should have a brief description of the measure and a link to its explanation
* Link the explanation in the UI so that a user can
* hover the measure name to see a brief description
* click on the measure name to get an explanation