update docs to reference 1.13.6 (#15182)

Author: techknowlogick

docs/config.yaml

@@ -18,7 +18,7 @@ params:
   description: Git with a cup of tea
   author: The Gitea Authors
   website: https://docs.gitea.io
-  version: 1.13.3
+  version: 1.13.6
   minGoVersion: 1.14
   goVersion: 1.16
   minNodeVersion: 10.13