Merge branch 'link-to-usage-plots' into 'master'

Add link to KPI plots (preliminary)

See merge request !327

documentation.md

@@ -18,6 +18,9 @@ Some frequently used documentation links are listed below:
 * Guidelines for [Service Providers](https://hifis.net/doc/backbone-aai/guidelines-services/)
 * How to [use HIFIS Transfer Service](https://hifis.net/doc/core-services/fts-endpoint/)
 
+#### Service Usage
+* [Exemplary usage plots](https://gitlab.hzdr.de/hifis/overall/kpi/kpi-plots-ci/-/blob/master/example_graphs.md) for AAI usage, Helmholtz Codebase Service, and HIFIS Software Course Participation. More to come!
+
 #### Documentation on the Current Status of Implementation
 
 * [List of Services in Cloud Service Portfolio: Pipeline and Current Catalogue](https://hifis.net/doc/service-portfolio/service-portfolio-management/current-services-in-portfolio/)