Link properly API keys from User and roles authentication docs

[eedugon]

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