ref(docs): update section names; revamp dev docs

This PR changes the following documentation section titles:

"Understanding Deis" -> "About"
"Installing Deis" -> "Installation Guide"
"Using Deis" -> "User Guide"
"Managing Deis" -> "Operations Guide"

Using "Deis" in the title seemed redundant, as we are already on the Deis documentation. There are also some user guide changes as well, such as making commands link properly to the reference guide. More changes to come soon (including some more refactoring of the user guide to push some of the docs to the reference guide), but wanted to start a discussion while this work is ongoing. Critique away! :)