Remove documentation from popit repository

This now lives in the https://github.com/mysociety/popit.poplus.org repo
Update links to point to popit.poplus.org
Change docs app to redirect to new documentation

Instead of rendering markdown we simply redirect to the new
documentation on popit.poplus.org. Because the structure of the
documentation has changed quite a bit we just redirect to the root of
the new documentation rather than to specific pages.
Update info app to redirect to popit.poplus.org
Redirect production site to the documentation site

The documentation then does a decent job of explaining what PopIt is and
why you might want to use it. It also has links to create a new
instance.
Merge pull request #454 from mysociety/446-move-documentation

Remove documentation now in popit.poplus.org repo