ref(installation): rename installing_deis

One less word in the title makes it a bit more clear. Less is more!
ref(user_guide): rename using_deis

Makes the name more clear for users as well as removing redundancy.
They're already on the Deis documentation, so "user guide" makes more
sense.
ref(operations_guide): rename managing_deis

We are on the Deis docs, so using "Deis" in the title is a bit
redundant. Switching the title to "Operations Guide" reflects what the
documentation is trying to explain.
ref(about): rename understanding_deis

Using "Deis" in the title while inside the Deis documentation is
redundant. Changing the title to "About" better reflects what this
section is describing.