[TASK] Update contribution walkthrough link

The contribution walkthrough has been moved from the
wiki to docs.typo3.org

Change-Id: I683bfad410aa75a1dab7fee91b7f0b3cee60070d
Resolves: #77423
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/49405


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Tobias Adolph <mail@tobiasadolph.de>
Tested-by: Tobias Adolph <mail@tobiasadolph.de>
Reviewed-by: Stefan Bürk <stefan.buerk@impactmedia.de>
Tested-by: Stefan Bürk <stefan.buerk@impactmedia.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>