Clickable links to related readonly M2M instances in the admin

### Code of Conduct

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

### Feature Description

The feature is a small change in the `AdminReadonlyField`'s `contents` method: Instead of joining the string representation of the related objects, join the admin_url's of them, just like with the ForeignKeys (or OneToOneFields).

### Problem

When a ManyToManyField is readonly in a ModelAdmin, a list of the string representations of the related objects is shown. That isn't very useful, the idea would be to have a clickable link to be able to view the related object.

### Request or proposal

proposal

### Additional Details

_No response_

### Implementation Suggestions

In the `AdminReadonlyField.contents` method, when the field is a many to many relationship and the value is set, the code is as follows:
```python
if isinstance(f.remote_field, ManyToManyRel) and value is not None:
result_repr = ", ".join(map(str, value.all()))
```

The change would leave the code similar to this:
```python
if isinstance(f.remote_field, ManyToManyRel) and value is not None:
result_repr = "\n".join(
map(
lambda obj: self.get_admin_url(f.remote_field, obj),
value.all(),
)
)
```