Show the user a message inside a flow

## What

“show the user a message in a flow” (affirmation stage vs simple-message stage).

## Want

- “show the user a message in a flow” (affirmation stage vs simple-message stage)

## Ideas this came from

- [#13337 [FEATURE] affirmation stage](https://github.com/goauthentik/authentik/issues/13337) — Consider the situation of user enrollment that should be affirmed by someone in charge. After sign up I want to choose someone in charge to affirm me (a guest c
- [#23420 Add "Simple message" stage](https://github.com/goauthentik/authentik/issues/23420) — Add a stage that allows admins to show simple messages to users during a flow. Stage name is work in progress. Multiple configurable fields would be a plus So w

Start by reading the linked issues #13337 and #23420 to understand the proposed affirmation and simple-message stages. The issue names no files or tests; clarify the intended stage behavior, configuration, and completion criteria before locating the implementation.

Written by the indexing model from the issue text.