Add environment keyword to pages job Gatsby template project

See more in Add `environment` to gitlab project templates (&8588 - closed)

Implementation guide

• Add the environment keyword to the pages job in https://gitlab.com/gitlab-org/project-templates/gatsby/-/blob/master/.gitlab-ci.yml#L29

pages:
  environment: production

• Follow https://docs.gitlab.com/ee/development/project_templates.html#create-a-merge-request-with-vendor-details (you only need steps 1 & 4-6 since the project should already be vendored)