XATTR documentation navigation needs to be moved/merged

The Extended Attributes section needs to be moved down or possibly combined with another section. This is a secondary feature intended to be used by framework developers. The UX interviews we did a while back showed that having a long LHS menu impeded comprehension.

There are three options as I see it. One would be to combine it with Sub-Document operations since they are in fact Sub-Document operations. Or, we could combine it with Non-JSON documents, which is also an advanced feature. Finally, we could just have it in a section before Non-JSON, at the cost of another menu item.

The other related thing there is that it's not in keeping with the way the navigation is intended to work. If we are going to have an overview and language specific page (the examples), then the overview page should be a single page rooted at sdk. I think given that the content on each of these is short though, we should just combine them into the same page. Then it can be language specific. Let @amarantha-k or myself know if you need more info.