Feature Overview (aka. Goal Summary)

Currently, `signInPage` is a frontend config that controls the auth providers that show up in the RHDH sign-in page. However, its use is redundant and confusing:

• Configured Auth providers are defined under `auth.providers`, we can just read from this list. Configuring this and `signInPage` is redundant. 
• The guest provider does not obey this config, but is determined by `auth. environment` (see code). This is not intuitive and confusing for customers.

Goals (aka. expected user outcomes)

Streamline the auth setup by deprecating the `signInPage` config and automatically populating the sign-in page based on the defined provider(s) under `auth.providers`.

Requirements (aka. Acceptance Criteria):

• Remove `signInPage` entirely
  • Another option is to transition it to an optional/dev-only override to prevent breaking existing setups
    • Would need to understand the use cases/ need to keep this
• Deprecation notice for customers in release notes
• Update E2E tests: currently we use it to switch between configured providers (here for example).
• Update product docs
• Ensure the dynamic plugin signInPage override maintains its functionality

Out of Scope (Optional)

High-level list of items that are out of scope.

Customer Considerations (Optional)

Deprecation notice required

Documentation Considerations

Updates to product docs under Authentication in Red Hat Developer Hub