Feature Page component

This adds support for a `Page` entity, which should be a minimal CMS implementation.

Currently a single `RouteLoader` is available and should be defined in your `routing.yml` as follows:
```yml
my_cms_route_name:
    resource: .
    type: elcodi_page_render
```

Everything else is made configurable by the `elcodi_page` key.
```yml
elcodi_page:
    mapping: ~ # usual mapping setup
    routing:
        loader:      elcodi.core.page.router.simple_loader.loader
        route_name:  elcodi_page_render_view
        route_path: '/{path}'
        controller:  elcodi.core.page.controller.page:renderAction
```

TO DO:
- [x] add `replace` entries in `composer.json`.
- [x] ease route configuration.
- [x] multiple implementations of `RouteLoader`. (moved to https://github.com/elcodi/elcodi/issues/389)
- [x] gather feedback for changes.
- [x] add documentation.
Not defined "per se" in the standard, so we can do as we please, but I can't find a counterexample in Symfony's codebase. There's no use of classes from the global namespace with the "use" clause.