added docs for the core team
feature #3777 added docs for the core team (fabpot)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3777).

Discussion
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added docs for the core team

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | n/a
| Applies to    | all
| Fixed tickets | n/a

The Symfony Core Team is back!

Commits
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d734600 added docs for the core team