removed references to documentation from external sources
Removed more references to third party docs
removed more references to documentation from external sources
minor #4106 removed references to documentation from external sources (fabpot, WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

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.

Commits
-------

2cb8d27 removed more references to documentation from external sources
c39ea1d Removed more references to third party docs
0e2c491 removed references to documentation from external sources