Add link to official documentation

Author: BenediktSeidl

src/plugin.json

@@ -16,6 +16,7 @@
       "large": "img/checkmk_logo.svg"
     },
     "links": [
+      { "name": "Documentation", "url": "https://docs.checkmk.com/latest/en/grafana.html" },
       { "name": "GitHub", "url": "https://github.com/Checkmk/grafana-checkmk-datasource" },
       { "name": "Checkmk", "url": "https://Checkmk.com" }
     ],