Docs - Update authentication section to reflect current auth model

[jenna-winkler]

Problem

The deployment guide's basic authentication section (#basic-authentication) is outdated and actively misleading. It describes an admin password configuration that no longer exists. Auth is now binary - either disabled entirely (AUTH__DISABLE_AUTH=true) or OIDC-based (Keycloak or external OIDC provider). There is no "partial auth" or standalone basic auth option. Users attempting to follow the current docs hit a dead end.

What needs to change

Remove or replace the basic authentication section. The updated docs should clearly state upfront that the two supported auth modes are: auth disabled, or OIDC. It should explain when each is appropriate and link to the correct configuration for each path. Any references to configuring an admin password independent of Keycloak should be removed.

Context

A user attempted to deploy without Keycloak by following the basic auth section and was unable to proceed. Radek confirmed the section is stale and no longer reflects how auth works.