website points to old repo #257

It's clear after reading that the current location of the offlineimap project is the offlineimap3 repo, which installs a program named offlineimap (8, that's in py3). However, https://www.offlineimap.org/ does not say this clearly and points to the offlineimap (no commits in 3 years, py2 version).

This ticket is about fixing the website so that people that arrive understand what's going on precisely and quickly.