New cookbook recipe stub: Turn Doctrine repository into service

I remember stumbling upon some blog post that said: well, just make the repository a service and you'll be good. Somebody else will find this surely usefull.
Since I am not experienced with the rst-format yet the code sections could need some love.

Kind regards.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets |