Rewrite the puppet overview page

The idea here is to give a broad picture of what working with Puppet
is like.
Rewrite manifests & modules page

to include a capsule overview of the language, with contextual references.
Add diagrams of Puppet's data sources
Write new "Assigning Configurations to Nodes" page in Puppet section
Rearrange "next" links in Puppet section
Rearrange Puppet pages in PE3 sidebar
Revise the console "Grouping and Classifying" page

This page had a lot of info that properly belonged in the Puppet section. Now
that it has a home there, we can just link to it here.
Merge branch 'pe3_node_classification'