[Docs] Add developer documentation for Authorization Service

[jportner]

Inspiration: #126238 (comment)

We currently have a single dev docs page that describes Kibana's role-based access control (RBAC): https://www.elastic.co/guide/en/kibana/current/development-security.html

The security plugin also exposes an Authorization Service to consumers which can be used to check privileges manually, but it isn't mentioned in our dev docs, and its TSdocs are very sparse.

We now have more consumers relying on this service, we should add comprehensive developer documentation for it (in particular, how to obtain "privilege action strings").

[elasticmachine]

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