Follow-up from "Project integration management docs FKA Instance-level integrations"

The following discussions from !39216 (merged) should be addressed:

• @leipert started a discussion: (+1 comment)

  @mikelewis I think we need to link it in the docs navigation. Maybe we should link it like https://docs.gitlab.com/ee/user/project/integrations/services_templates.html

• @leipert started a discussion:

  Question: Should we link from the Service templates?