Ensure `environment` keyword is defined on all `deploy` jobs in documentation (batch 3)

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/ci/testing/browser_performance_testing.md
162:  stage: deploy

doc/ci/testing/load_performance_testing.md
181:  stage: deploy

doc/ci/examples/end_to_end_testing_webdriverio/index.md
227:  stage: deploy

doc/ci/variables/index.md
590:  stage: deploy
614:  stage: deploy
621:  stage: deploy
627:  stage: deploy
634:  stage: deploy
642:  stage: deploy

doc/ci/examples/deployment/index.md
46:  stage: deploy
65:  stage: deploy
90:  stage: deploy
98:  stage: deploy