I won't start for another few weeks. They have mentioned that the time to get to know their system should be about 3 months, but I want to get as good a start as possible.
You will have time to learn about their system when you will be working there. But if you want to get ahead of work by starting to look it up now, you might look like you are really involved in this new job and want to do well.
I don't know if they have or will make you sign a non disclosure agreement but it seems like you could not have access to these documents before your contracts starts as you will not have the permission to do so.
Is documentation something I could ask for?
Maybe the best approach is to ask for documentation in general and see what they can give you rather than wanting a specific document that you might not be able to get and that might make you look suspiscious to why you want access to those documents.
And seing that from another perspective, would the documents they would send you really give you a very detailed description of the system they are using ? And if so wouldn't you need to be in the company to analyse the documents so that you can ask questions ?
I don't see this document request as added value they would just send you something too easy or something you can't really process and that might get you more confused.
Congratulations for the job ! You seem overly exited about it ! Don't try to get ahead of your workload too fast. It is good to be motivated but don't stress yourself on how you will be able to fit with the team you are going to integrate.