Remove documentation now in popit.poplus.org repo

- Remove `docs/` directory
- Remove info pages
- Update documentation and info links to point to popit.poplus.org
- Change docs app to redirect to new documentation
- Change info app to redirect to new info pages
- Remove related files that are no longer needed
- Redirect popit.mysociety.org homepage to popit.poplus.org

Fixes #446