Update Maven site links for User doc and Download paths

The codehaus links still exist, but appear to be very old because they reference geotools 2.x
Fix javadoc link too

The old one was 404'ing.
Update About page latest dev and stable releases

Basing this on the GeoTools v11.3 blog post that says "the GeoTools 11 Stable series recommended for production systems"
Merge pull request #576 from leif81/patch-2

Update About page latest dev and stable releases
Merge pull request #575 from leif81/patch-1

Update Maven site links for User doc and Download paths