Fix ex_doc warnings for hidden module references

[sgiath]

Problem

mix check --fix emits multiple ex_doc warnings because usage-rules.md, lib/auth.ex, and lib/auth/profile.ex reference hidden modules such as SgiathAuth.Controller, SgiathAuth.Scope, SgiathAuth.Token, and SgiathAuth.Supervisor.

Why this matters

The warning noise makes it harder to spot new documentation regressions.

Suggested fix

Either unhide those modules for docs generation or update references to avoid hidden-module links.