removed references to documentation from external sources

| Q             | A
| ------------- | ---
| Doc fix?      | n/a
| New docs?     | n/a
| Applies to    | all
| Fixed tickets | n/a

Referencing external doc sources was a mistake as the versioning can be (and is) different from the versioning of the Symfony docs.

**WARNING** I will merge this one myself to coordinate the merge with the release of the new frontend on symfony.com.

I'm sorry, my fault: The `:doc:` prefix should be removed and they both have to be suffixed with `_`