improve install hint placing

[tooomm]

Moved from flathub-infra/flathub.org#54

The install hint Make sure to follow the setup guide before installing placement could be improved within the app info pages for better visibility and logical structure:

• On the right of the Install button or directly below it, not below the picture:
  

• Above the instructions since a user has to check that first :)
  

[allanday]

This might be obsoleted by #76, depending on who gets to which issue first.