Refactoring configuration docs

[Andrew Schwartzmeyer]

Hi all,

I'm going through and updating the CMake docs, and that brought me to 
our docs/configuration.md file. This file is huge! In the style of Jie's 
refactor of the isolator docs, I'd like to create a docs/configuration 
folder, and split these files into mesos.md, master.md, agent.md, 
libprocess.md, autotools.md, and cmake.md within that folder.

I think this important for discoverability, especially considering that 
we lack a table of contents. After re-organizing, I would suggest that 
docs/configuration.md become a table of contents with relative links 
into the configuration folder.

Any objections / ongoing work I might interfere with?

Thanks,

Andrew Schwartzmeyer