[WIP] ref(docs): doc changes

This PR changes the following documentation section titles:

"Understanding Deis" -> "About"
"Installing Deis" -> "Installation Guide"
"Using Deis" -> "User Guide"
"Managing Deis" -> "Operations Guide"

This is in reflection to https://github.com/deis/deis/issues/1050#issuecomment-45805677

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, giving the autodocs some love.

The client docstrings have also been refactored to make them more portable and easier to read on http://docs.deis.io/en/latest/reference/client/. One breaking change was introduced here, which was to the `clusters:create` docstring. See 0080a07 for more information. The angle brackets on parameters were removed, since docopt recommends CAPITAL LETTERS as parameters for docstrings. See https://github.com/docopt/docopt/blob/master/examples/arguments_example.py

More changes to come soon:

- [ ] push some of the user guide to the reference guide
- [ ] update some of the old references to Chefland in the contributing guide

Critique away! :)