minor #6367 Simplified the contribution article for Symfony Docs (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #6367).

Discussion
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Simplified the contribution article for Symfony Docs

After last Saturday's Hack Day, I realized that we could simplify the documentation of the contribution process significantly. I'll propose changes to the code contribution, but let's start with the doc contribution.

Commits
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eeb6abb Simplified the contribution article for Symfony Docs

Author: xabbuh