Add FAIR Mappings schema link to mkdocs.yml

Author: matentzn

mkdocs.yml

@@ -81,6 +81,7 @@ nav:
           - DCAT-AP-Schema.org: case-study/case-study-dcat-ap-schemaorg.md
           - ARIADNE mappings: case-study/case-study-ariadne.md
   - FAIR Mapping recommendations: recommendation.md
+  - FAIR Mappings schema: https://mapping-commons.github.io/fair-mappings-schema/
   - Decision log of the Working Group: decision-log.md
   - Related efforts: related-efforts.md
   - Resources about FAIR mappings: resources.md