feat: fallback / catch all route for ion-router

### Prerequisites

- [X] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#creating-an-issue).
- [X] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [X] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

This is a follow up for #18687.

The only way to do a fallback route as of today is to use `url="/:any"` :
```html

```

This "trick" is not documented.

### Describe the Use Case

Most app need to display a 404 page or redirect the user to a sensible page when the url is not found.

### Describe Preferred Solution

`` would be nice as it would be consistent with with ion-route-redirect [usage of *](https://ionicframework.com/docs/api/route-redirect#usage).

Whatever solution is picked should be documented as it is a common use case.

### Describe Alternatives

There are several solution:

1. Keep the current `` but document it
2. Use `` this would be consistent with ion-route-redirect [usage of *](https://ionicframework.com/docs/api/route-redirect#usage)
3. Use `` (i.e. no url)

### Related Code

_No response_

### Additional Information

I'm up to help implement and/or document this once a solution is picked.

What do you think ?

Bonus question: what should we do when multiple catch all routes are defined ? My preference would be to trigger an error as there should be no reason to be in this situation. Note that as of today the first catch-all route would be used but the current behavior seems brittle as it is mostly an implementation detail and not covered by tests.