Added some overview docs on extensions and management.
Review Request #3635 — Created Dec. 6, 2012 and submitted
Added some overview docs on extensions and management. These docs are somewhat basic, but give a summary of what extensions are, some basic info on installing them, and what to expect on the Extensions page. We can probably fill these out more later, but there's not really a ton to say at this point in time anyway.
Built the docs and read through them.