Make sure that all EE docs/headings have the EE tiers assigned to them

We recently introduced product badges that makes it easy for users to check what features are in EE tiers.

We should make sure all docs have the appropriate tier associated to them.

TODOs:

• Check the features and their accompanied docs in https://about.gitlab.com/features/ and find which ones don't have the tier defined. The source features.yml might help.
• Check https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/license.rb as well.