[TYPO3-doc] Mentioning Github in Feedback section of each manual

[François Suter]

> Today turned out to be pretty busy. I did for all the Guides and
> Tutorials, but not yet for the References (except TSconfig, which was my
> test case).

Done also for the references. I updated only the ones which are actively 
maintained. For the others, the work can be done when and if anyone 
picks them up.

Cheers

-- 

Francois Suter

Work: Cobweb Development Sarl - http://www.cobweb.ch

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