Remove all references to ZenHub

[filipefurtad0]

What we should change and why (this is tech debt)

We need to remove all references to Zenhub from our documentation:

• https://github.com/openfoodfoundation/openfoodnetwork/wiki/ -> done here.

• Testing Handbook -> done here:
  

• Main repo https://github.com/openfoodfoundation/openfoodnetwork -> done here.

• OFN handbook

• ...am I missing any?

Context

Replacing Zenhub with GH Actions board.

Impact and timeline

[sigmundpetersen]

Hi @filipefurtad0 👋 Do you think we can close this?

[filipefurtad0]

Yes thanks @sigmundpetersen :-)
Sorry, lost track of it! Closing.