Rework Architecture Images

We got multiple images that almost look identically, e.g. `docs/architecture/assets/Anwendung_zentraleGUI_PermissionService.png`. What purpose does `Identity Provider` have here? Its just floating around. ;)

_Originally posted by @devtobi in https://github.com/it-at-m/refarch/pull/940#discussion_r3664444704_

## Problems
- https://refarch.oss.muenchen.de/architecture/frontend-architecture.html#centralized-frontend : Frontend part of gateway instead of current default separate service
- Images with no background and black text not readable in darkmode
- https://refarch.oss.muenchen.de/architecture/external-access.html
- https://refarch.oss.muenchen.de/architecture/security-flows.html