[Dev Docs] Provide setup instructions for configuring authentication realms

[legrego]

Providing docs for configuring and using our various authentications realms would enable us and other teams to be more self-sufficient when they need to test a specific setup or user flow.

These docs should be targeted at Kibana contributors, as they should be curated for a local testing environment, and not a production-ready installation.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)