Added some overview docs on extensions and management.

Review Request #3635 — Created Dec. 6, 2012 and submitted — Latest diff uploaded

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Review Board
release-1.7.x

Reviewers

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.