Add 3.5 links on the front-page and old reference

Previously there was no way to navigate to the new Puppet 3.5
manual from the main doc index, nor a way to hop over to it
from the 3.0 manual (but /3/ did have a link to /3.5/).
(DOC643) 2.8.x agents not compatible with 3.x masters
Merge pull request #306 from ahpook/3_5_links

Add 3.5 links on the front-page and old reference