Mention interpolation of vardir and confdir in puppet.conf
Add page on config file environments
Add more information to directory environments page

Stuff like accessing environment from manifests, info about the
HTTP api, and more.
Rearrange the 3.5 nav sidebar a bit

I was sleepwalking a bit when arranging these sections. Language should go
before a bunch of that other stuff, references should be higher and not have the
word "generated" in it (because who cares), the type reference may as well have
its own top-level item because it's just that important, and a few sections
should talk about what's included in them so readers know whether they want to
click.