Ability to configure an initial page to open

### Clear and concise description of the problem

Right now when Histoire is opened, a blank screen is show.

### Suggested solution

I'd be nice to be able to have a **Welcome page/Home page** that you can redact yourself (kind of an `index.html` or something).

### Alternative

A good alternative would be to have a way to indicate which Story you'd like to be opened by default.

### Additional context

_No response_

### Validations

- [X] Follow our [Code of Conduct](https://github.com/histoire-dev/histoire/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [Contributing Guidelines](https://github.com/histoire-dev/histoire/blob/main/CONTRIBUTING.md).
- [X] Read the [docs](https://histoire.dev/guide/).
- [X] Check that there isn't [already an issue](https://github.com/histoire-dev/histoire/issues) that requests the same feature to avoid creating a duplicate.

The issue does not name files, tests, or an entry point. First trace how Histoire chooses and renders its initial screen, then determine whether configuration should select a custom welcome page, a default story, or both. Done means opening Histoire no longer shows a blank screen when an initial destination is configured.

Written by the indexing model from the issue text.