Getting started with Doctrine 2 and CodeIgniter
I'll start off by saying that no, I don't know of any decent guides for getting started on Doctrine. If you understand how to use PHP as an object-oriented language (classes, inheritance, namespaces, etc) then you should have no trouble learning Doctrine 2 because it has been designed in such a way that you are simply creating and accessing regular PHP classes. You are absolutely right - reading the documentation is (in my opinion) the best place to start with Doctrine.
You could start by creating simple a simple system like a blog; this should help you get to grips with how Doctrine handles associations and give you a chance to use the command line utility. When you feel like you need more functionality or you're stuck on something, the documentation will almost always be enough to help you out and if not, SO has thousands of Doctrine-related questions & answers.
Just remember, Doctrine 2 is not supposed to be complicated. It is supposed to be a transparent layer that lets you access your database as PHP classes.