OK, I understand. I also have struggled with this a bit because I like to link to the reference content to supplement my answers and I'm accustomed to having the top level reference pages accessible at a single click.
As the content on arduino.cc has grown and diversified, I think the designers had more and more difficulty fitting it in to the old interface. So now arduino.cc has essentially been broken into multiple websites. These share the same navigation style, but the navigation provides only content relevant to that site.
The sites are accessed via the links at the top left of the website:
Documentation content is all stored under the "Arduino" website, so the navigation path from the forum to the reference is:
- Click the "ARDUINO" link (shown as "ARDUINO.CC" on larger windows).
- Hover over the "DOCUMENTATION" tab.
- From the dropdown menu, click on "REFERENCE".
After that, you might want to get back to the forum. It is accessed under the "COMMUNITY" tab.