Docs: Add a link to GitLab's Sentry install via CI docs in 'deploying sentry' section

@kencjohnston said:

Could someone add a docs update to add a link to https://docs.gitlab.com/ee/user/clusters/applications.html#install-sentry-using-gitlab-ci from https://docs.gitlab.com/ee/user/project/operations/error_tracking.html#deploying-sentry

https://gitlab.slack.com/archives/CJR3CMUKB/p1579278080001900