Link properly API keys from User and roles authentication docs #2201
Description
User and Roles -> Cluster -> User authentication includes a child called token-based authentication services: https://www.elastic.co/docs/deploy-manage/users-roles/cluster-or-deployment-auth/token-based-authentication-services.
In that page, the API-key has a link to the API, but it's lacking a link to the narrative docs about API keys: https://www.elastic.co/docs/deploy-manage/api-keys/elasticsearch-api-keys
Action
Include links in both directions, indicating the purpose of the API key in the context of elasticsearch in the API keys doc