Admin : Register Custom Pages and routes via an Admin URL resolver

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Have an abstract view `AdminPage` (like `ModelAdmin`) that can be used to register Views on the admin and leverage the admin's default permission model.

These views should then be wired up to an `adminUrlPatterns`. This should make it significantly easier to build custom dashboards using Django without having to reach for external options like React-Admin and rewiring the APIs all over again

### Problem

Django's admin is one of its most distinguishing features.

In the past, I attempted to leverage the admin interface to build internal pages, as it provided many of the features typically required for an internal site. However, it quickly became clear that Django's admin was designed primarily for database management and not for broader site functionality.

As a result, we were compelled to build a separate admin dashboard using other front-end technologies, such as React, on top of the existing API.

Building able to register custom pages easily would save a lot of dev time for smaller teams and independent devs.

### Request or proposal

request

### Additional Details

The [existing ticket](https://code.djangoproject.com/ticket/18665) on Trac points to AdminPlus, which aims to just add a view and hook it into the index under an app.

This proposal intends to put the `AdminPage` on equal footing and not need to be registered under an app to exist. It should be able to exist under its own heading or fall back to some default (like 'Pages')

This should make it easier for packages that offer a custom dashboard experience to build on top of django-admin instead of recommending abandoning the core feature completely, or having two separate backoffices (like wagtail) instead of just hiding some views with appropriate perms

### Implementation Suggestions

_No response_

Start by reviewing the existing ticket #18665 and AdminPlus, then compare their approach with the requested standalone AdminPage and adminUrlPatterns. A complete proposal would need to define registration, URL wiring, permission behavior, and heading or fallback behavior; the issue does not identify files, tests, or a verification path.

Written by the indexing model from the issue text.