add Couper devcontainer feature to the index

Author: malud

_data/collection-index.yml

@@ -1138,3 +1138,8 @@
   contact: https://github.com/navinkrishnan/devcontainer-templates-btp/issues
   repository: https://github.com/navinkrishnan/devcontainer-templates-btp
   ociReference: ghcr.io/navinkrishnan/devcontainer-templates-btp
+- name: Couper DevContainer Feature
+  maintainer: Marcel Ludwig (malud)
+  contact: https://github.com/coupergateway/features/issues
+  repository: https://github.com/coupergateway/features
+  ociReference: ghcr.io/coupergateway/features/couper