Assetic fixes

Two additions to hopefully help newcomers:

- The `bundles` option in the Assetic configuration defaults to `[]`, it must be changed, one way or another;
- Relative paths in CSS files don’t work anymore and the `cssrewrite` filter [does not work with the `@AsseticFooBundle` syntax][comment], so a workaround is proposed.

[comment]: https://github.com/kriswallsmith/assetic/issues/53#issuecomment-2976018

I have not included the other formats besides YAML for the config file because I’m not comfortable enough with them and the AsseticBundle Configuration Reference only includes YAML. I have not tested the PHP version of the example for the output parameter.

Of course, I will gladly update this based on suggestions.