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

[François Suter]

> Thanks for the feedback. I'll proceed, since I have planned the time for
> it today.

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).

Cheers

-- 

Francois Suter

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

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