Switch to new User Guide

[vladmihalcea]

The documentation build process should use the new User Guide. We need to change the hibernate.org site to use a new URL, without en-US like it's the case for topical guides.

[sebersole]

This renders too much still. I went ahead and tackled this slightly differently. I am also removing the handling of the symlinks; we can simply handle that manually whenever the "next release" becomes the "current" stable release.

Afterwards I will clean the 5.1 dir from the doc server and re-publish