Add some docs (#198)

docs/administration/healthcheck.md

@@ -0,0 +1,13 @@
+# Healthcheck
+
+A healthcheck is a simple HTTP GET request to the `/healthcheck` endpoint. It returns a `200 OK` response if the server is healthy.
+
+## Example
+
+```shell
+curl http://localhost:6157/healthcheck
+```
+
+```json
+{"database":"ok","opengist":"ok","time":"2024-01-04T05:18:33+01:00"}
+```