[TYPO3-doc] cross linking between different documentations

[Christian Müller]

Hi documentation team,

I came across the need to link between two different documentations in
Flow and found your Intersphinx configuration. Would it make sense to
add our documentations to that, at least I think it is needed to provide
interlinking, right?
I would prepare a change for the yml file then.

Cheers,
Christian