Ensure `environment` keyword is defined on all `deploy` jobs in documentation (batch 6)

See Ensure `environment` keyword is defined on all ... (&8590 - closed) for more details

Implementation guide

Make sure that all stage: deploy jobs in the following documentation files are already have environment: keyword.

If some of them don't, then add the

environment: production

(substitute production to other environmens like dev or staging where it makes more sense).

doc/user/project/releases/index.md
311:  stage: deploy

doc/user/project/clusters/multiple_kubernetes_clusters.md
56:  stage: deploy
63:  stage: deploy

doc/user/project/pages/introduction.md
308:  stage: deploy

doc/user/project/pages/getting_started/pages_from_scratch.md
259:  stage: deploy
282:  stage: deploy
337:  stage: deploy
381:  stage: deploy

doc/user/packages/conan_repository/index.md
307:  stage: deploy

doc/user/packages/composer_repository/index.md
144:     stage: deploy

doc/user/packages/container_registry/index.md
299:  stage: deploy