Documents help finished for 3.12

[Ekaterina Gerasimova <kittykat3756 gmail com>]

Hi all, I have finished working on gnome-documents for 3.12. The
existing strings should not be changed and I think it is very unlikely
that new strings will be added, although a few pages do still need to
be written.

The Documents help used to be in gnome-user-docs and is now in
gnome-documents itself. I did make sure to move the translations
across as well, so it should be mostly translated already.
Unfortunately, it had a fair few UI changes, so many of the
instructions needed minor updates.

I have a feeling that 3.12 will be the best documented release in a
while, so it would be pretty awesome if it could also be the best
translated! :)