Update docs/config.yaml to 1.15.6 (#17472)

Update the version in docs/config.yaml to 1.15.6

Signed-off-by: Andrew Thornton <art27@cantab.net>

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: 1.15.5
+  version: 1.15.6
   minGoVersion: 1.16
   goVersion: 1.17
   minNodeVersion: 12.17